docs: Remove unsupported payloadSizeThreshold from JSON stdin config (#808)

Documentation incorrectly showed `payloadSizeThreshold` as a supported
field in JSON stdin gateway configuration. The field exists in
`GatewayConfig` (TOML format) but is missing from `StdinGatewayConfig`
struct.

## Changes

- **Removed from JSON example**: Deleted `payloadSizeThreshold: 1024`
from gateway config example
- **Removed from field documentation**: Eliminated field description
from Gateway Configuration Fields section
- **Added configuration alternatives**: New section clarifying users
must use:
  - CLI flag: `--payload-size-threshold <bytes>`
  - Environment variable: `MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD=<bytes>`

## Before

```json
{
  "gateway": {
    "port": 8080,
    "payloadDir": "/tmp/jq-payloads",
    "payloadSizeThreshold": 1024  // ❌ Not supported in JSON stdin
  }
}
```

## After

```json
{
  "gateway": {
    "port": 8080,
    "payloadDir": "/tmp/jq-payloads"
  }
}
```

**Configuration via CLI/env still works:**
```bash
awmg --config-stdin --payload-size-threshold 2048 < config.json
# or
MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD=2048 awmg --config-stdin < config.json
```

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `example.com`
> - Triggering command: `/tmp/go-build3919639669/b275/launcher.test
/tmp/go-build3919639669/b275/launcher.test
-test.testlogfile=/tmp/go-build3919639669/b275/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true rtcfg --local
x_amd64/compile user.email` (dns block)
> - `invalid-host-that-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build3919639669/b263/config.test
/tmp/go-build3919639669/b263/config.test
-test.testlogfile=/tmp/go-build3919639669/b263/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true 64/src/runtime/cgo
64/src/unicode/casetables.go ache/go/1.25.6/x64/pkg/tool/linu-o
credential.usern/opt/hostedtoolcache/go/1.25.6/x64/pkg/tool/linux_amd64/vet`
(dns block)
> - `nonexistent.local`
> - Triggering command: `/tmp/go-build3919639669/b275/launcher.test
/tmp/go-build3919639669/b275/launcher.test
-test.testlogfile=/tmp/go-build3919639669/b275/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true rtcfg --local
x_amd64/compile user.email` (dns block)
> - `slow.example.com`
> - Triggering command: `/tmp/go-build3919639669/b275/launcher.test
/tmp/go-build3919639669/b275/launcher.test
-test.testlogfile=/tmp/go-build3919639669/b275/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true rtcfg --local
x_amd64/compile user.email` (dns block)
> - `this-host-does-not-exist-12345.com`
> - Triggering command: `/tmp/go-build3919639669/b284/mcp.test
/tmp/go-build3919639669/b284/mcp.test
-test.testlogfile=/tmp/go-build3919639669/b284/testlog.txt
-test.paniconexit0 -test.timeout=10m0s -test.v=true /go-build main
x_amd64/vet` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/github/gh-aw-mcpg/settings/copilot/coding_agent)
(admins only)
>
> </details>

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>


----

*This section details on the original issue you should resolve*

<issue_title>📚 Documentation Reconciliation Report - JSON Stdin Config
Missing payloadSizeThreshold Field</issue_title>
<issue_description>## Summary

Found **1 critical discrepancy** between documentation and
implementation during nightly reconciliation check (2026-02-07).

- Workflow Run:
[#21779574765](https://github.com/github/gh-aw-mcpg/actions/runs/21779574765)
- Date: 2026-02-07
- Branch: main

**Overall Assessment**: Documentation is **99% accurate** with excellent
quality. Only one configuration field is affected.

---

## Critical Issues 🔴

### 1. JSON Stdin Config Missing `payloadSizeThreshold` Field

**Location**: README.md lines 132, 201

**Problem**: Documentation shows `payloadSizeThreshold` as configurable
in JSON stdin format:
``````json
{
  "gateway": {
    "port": 8080,
    "payloadDir": "/tmp/jq-payloads",
    "payloadSizeThreshold": 1024  // ← This field is documented but not supported
  }
}
``````

**Actual Behavior**: The field is missing from `StdinGatewayConfig`
struct:
``````go
// internal/config/config_stdin.go:27-34
type StdinGatewayConfig struct {
    Port           *int   `json:"port,omitempty"`
    APIKey         string `json:"apiKey,omitempty"`
    Domain         string `json:"domain,omitempty"`
    StartupTimeout *int   `json:"startupTimeout,omitempty"`
    ToolTimeout    *int   `json:"toolTimeout,omitempty"`
    PayloadDir     string `json:"payloadDir,omitempty"`
    // PayloadSizeThreshold is MISSING
}
``````

**Impact**: 
- Users cannot configure payload size threshold via JSON stdin
configuration
- **Workaround exists**: Default value (1024 bytes) is automatically
applied via `applyDefaults()` in `config_payload.go:20-22`
- Alternative configuration methods work: CLI flag
`--payload-size-threshold` and env var
`MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD`

**Suggested Fix**:
1. Add field to `StdinGatewayConfig`:
   ``````go
   PayloadSizeThreshold *int `json:"payloadSizeThreshold,omitempty"`
   ``````

2. Update conversion in `convertStdinConfig()` (around line 154):
   ``````go
   if stdinCfg.Gateway.PayloadSizeThreshold != nil {
cfg.Gateway.PayloadSizeThreshold =
*stdinCfg.Gateway.PayloadSizeThreshold
   }
   ``````

**Code Reference**: 
- Missing in: `internal/config/config_stdin.go:27-34`
- Exists in: `internal/config/config_core.go:61` (GatewayConfig has the
field)
- Documented in: README.md lines 132, 201

---

## Important Issues 🟡

### 2. External Documentation Link May Be Inaccessible

**Location**: README.md lines 7, 105, 186

**Link**:
https://github.com/github/gh-aw/blob/main/docs/src/content/docs/reference/mcp-gateway.md

**Problem**: Link appears to reference a private repository
(`github/gh-aw`), which returns a GitHub login page instead of
documentation content.

**Impact**: External users cannot access the "Full Configuration
Specification" reference that is prominently linked three times in
README.md.

**Recommendation**: 
- If `github/gh-aw` is a private repository, consider mirroring
essential configuration documentation in this public repo
- Or clearly note that the link requires GitHub organization access
- Or provide an alternative public documentation source

---

## Documentation Completeness ✅

### Accurate Sections Verified

✅ **Docker Quick Start** (lines 32-77)
- Docker run command is correct and complete
- All required flags properly documented
- Environment variables match implementation
- Port mapping instructions accurate

✅ **TOML Configuration** (lines 85-102)
- Field names match code exactly
- Command/args pattern correctly explained
- Example configuration parses successfully

✅ **JSON Stdin Configuration** (lines 104-186)
- 99% accurate (only payloadSizeThreshold missing)
- Type field options correct (stdio/http/local)
- Container requirement correctly stated
- Command field restriction clearly documented
- Mount format validation matches implementation
- Environment variable expansion documented correctly

✅ **CLI Flags Documentation** (lines 214-243)
- **100% accurate** - All 15 flags verified
- Default values match source code
- Flag types correct
- Descriptions match implementation

✅ **Environment Variables** (lines 246-274)
- All 6 documented variables exist and work correctly:
  - `MCP_GATEWAY_PORT`
  - `MCP_GATEWAY_DOMAIN`
  - `MCP_GATEWAY_API_KEY`
  - `MCP_GATEWAY_LOG_DIR`
  - `MCP_GATEWAY_PAYLOAD_DIR`
  - `MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD`

✅ **Features List** (lines 9-27)
- All 7 claimed features verified in codebase:
  - Configuration Modes (TOML/JSON) ✓
  - Spec-Compliant Validation ✓
  - Variable Expansion ✓
  - Type Normalization ✓
  - Schema Normalization ✓ (`schema_normalization_test.go`)
  - Routing Modes ✓ (`routed.go`, `unified.go`)
  - Docker Support ✓
  - Stdio Transport ✓
  - Container Detection ✓ (`tty/container.go`)
  - Enhanced Debugging ✓
  - Per-ServerID Logs ✓

✅ **CONTRIBUTING.md**
- All make targets exist and work: `build`, `test...

</details>


> **Custom agent used: agentic-workflows**
> GitHub Agentic Workflows (gh-aw) - Create, debug, and upgrade
AI-powered workflows with intelligent prompt routing



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #802

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

Author: lpcox

README.md

@@ -128,8 +128,7 @@ For the complete JSON configuration specification with all validation rules, see
     "domain": "localhost",
     "startupTimeout": 30,
     "toolTimeout": 60,
-    "payloadDir": "/tmp/jq-payloads",
-    "payloadSizeThreshold": 1024
+    "payloadDir": "/tmp/jq-payloads"
   }
 }
 ```
@@ -198,13 +197,16 @@ See **[Configuration Specification](https://github.com/github/gh-aw/blob/main/do
   - Must be positive integer
 - **`payloadDir`** (optional): Directory for storing large payload files (default: `/tmp/jq-payloads`)
   - Payloads are organized by session: `{payloadDir}/{sessionID}/{queryID}/payload.json`
-- **`payloadSizeThreshold`** (optional): Size threshold in bytes for payload storage (default: 1024)
-  - Payloads **larger** than this threshold are stored to disk and return metadata
-  - Payloads **smaller than or equal** to this threshold are returned inline
-  - Common values: `512` (aggressive), `1024` (default), `2048` (balanced), `10240` (minimal storage)
 
 **Note**: Gateway configuration fields are validated and parsed but not yet fully implemented.
 
+**Configuration Alternatives**:
+- **`payloadSizeThreshold`** is not supported in JSON stdin format. Use:
+  - CLI flag: `--payload-size-threshold <bytes>` (default: 1024)
+  - Environment variable: `MCP_GATEWAY_PAYLOAD_SIZE_THRESHOLD=<bytes>`
+  - Payloads **larger** than this threshold are stored to disk and return metadata
+  - Payloads **smaller than or equal** to this threshold are returned inline
+
 **Environment Variable Features**:
 - **Passthrough**: Set value to empty string (`""`) to pass through from host
 - **Expansion**: Use `${VAR_NAME}` syntax for dynamic substitution (fails if undefined)