Add fstarmcp skill and MCP server config for Copilot CLI

Add two things for integrating fstar-mcp with Copilot CLI:

1. .copilot/mcp-config.json - Registers fstar-mcp as an HTTP MCP
   server on localhost:3001 so Copilot CLI auto-connects to it.

2. .github/skills/fstarmcp/SKILL.md - Skill documentation covering:
   - Server startup and MCP registration
   - API protocol (JSON-RPC 2.0 over HTTP)
   - Session lifecycle: create_session, typecheck_buffer, close_session
   - All available tools: lookup_symbol, get_proof_context, etc.
   - HaclPulse-specific configuration (include dirs, options)
   - Tips for incremental workflow, Pulse support

3. .copilot/.gitignore - Excludes user-specific files (logs, config)
   while keeping mcp-config.json in version control.

Tested end-to-end: session creation, error detection, incremental
re-typecheck, symbol lookup all work correctly with the server.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nikswamy

.copilot/.gitignore

@@ -0,0 +1,4 @@
+# Exclude everything except mcp-config.json
+logs/
+config.json
+session-state/

.copilot/mcp-config.json

@@ -0,0 +1,8 @@
+{
+  "mcpServers": {
+    "fstar-mcp": {
+      "type": "http",
+      "url": "http://localhost:3001/"
+    }
+  }
+}

.github/skills/fstarmcp/SKILL.md

@@ -0,0 +1,157 @@
+---
+name: fstarmcp
+description: Use the F* MCP server for interactive, incremental typechecking of F* and Pulse code
+---
+
+## Overview
+
+The F* MCP server (`fstar-mcp`) provides an HTTP API wrapping F*'s `--ide` protocol.
+It enables **incremental typechecking**: create a session once, then re-typecheck modified
+code without restarting F* or reloading dependencies. This dramatically speeds up iterative
+proof development.
+
+Binary location: `../fstar-mcp/target/release/fstar-mcp` (relative to the FStar repo root).
+
+## MCP Registration
+
+The server is registered in `.copilot/mcp-config.json` at the repo root as an `http` type
+server on `http://localhost:3001/`. Copilot CLI will auto-connect when the server is running.
+
+## Starting the Server
+
+The server must be started before Copilot CLI can use it:
+
+```bash
+# Start on port 3001 (matches mcp-config.json)
+FSTAR_MCP_PORT=3001 ../fstar-mcp/target/release/fstar-mcp &
+
+# With debug logging
+RUST_LOG=fstar_mcp=debug FSTAR_MCP_PORT=3001 ../fstar-mcp/target/release/fstar-mcp &
+```
+
+The server runs on `http://127.0.0.1:3001`. All API calls are JSON-RPC 2.0 POST requests to `/`.
+
+## API Protocol
+
+All requests use JSON-RPC 2.0 over HTTP POST to the root endpoint `/`:
+
+```bash
+curl -s -X POST http://localhost:3001/ \
+  -H 'Content-Type: application/json' \
+  -H 'Accept: application/json' \
+  -d '{"jsonrpc": "2.0", "method": "tools/call", "id": 1, "params": {"name": "TOOL_NAME", "arguments": {...}}}'
+```
+
+Responses contain `result.content[0].text` with a JSON string that must be parsed again.
+
+## Workflow
+
+### 1. Create a Session
+
+```bash
+curl -s -X POST http://localhost:3001/ \
+  -H 'Content-Type: application/json' \
+  -d '{"jsonrpc":"2.0","method":"tools/call","id":1,"params":{"name":"create_session","arguments":{
+    "file_path": "/path/to/Module.fst",
+    "cwd": "/project/root",
+    "include_dirs": ["/path/to/lib", "/path/to/specs"],
+    "options": ["--cache_dir", "/path/to/obj", "--already_cached", "Prims FStar"]
+  }}}'
+```
+
+**Parameters:**
+- `file_path` (string, optional): Path to the F* file. Must exist on disk. If omitted, creates a temp file.
+- `cwd` (string, optional): Working directory. Defaults to file's directory.
+- `include_dirs` (string[], optional): Directories for `--include`.
+- `options` (string[], optional): Extra F* CLI options.
+
+**Returns:** `session_id`, `status` ("ok"/"error"), `diagnostics[]`, `fragments[]`
+
+The initial `create_session` typechecks the file's current contents on disk. The returned
+`session_id` is used for all subsequent operations.
+
+### 2. Typecheck Modified Code
+
+```bash
+curl -s -X POST http://localhost:3001/ \
+  -d '{"jsonrpc":"2.0","method":"tools/call","id":2,"params":{"name":"typecheck_buffer","arguments":{
+    "session_id": "UUID",
+    "code": "module Foo\nlet x = 42",
+    "kind": "full"
+  }}}'
+```
+
+**Parameters:**
+- `session_id` (string, required): From `create_session`.
+- `code` (string, required): Full module source code. Must start with matching `module` declaration.
+- `lax` (boolean, optional): Shortcut for `kind: "lax"`.
+- `kind` (string, optional): `"full"` (default), `"lax"`, `"cache"`, `"reload-deps"`, `"verify-to-position"`, `"lax-to-position"`.
+- `to_line` / `to_column` (integer, optional): For position-based kinds.
+
+**Returns:** `status`, `diagnostics[]`, `fragments[]`
+
+Each fragment has `start_line`, `end_line`, `start_column`, `end_column`, and `status` ("ok"/"failed").
+
+### 3. Look Up Symbols
+
+```bash
+curl -s -X POST http://localhost:3001/ \
+  -d '{"jsonrpc":"2.0","method":"tools/call","id":3,"params":{"name":"lookup_symbol","arguments":{
+    "session_id": "UUID",
+    "file_path": "/path/to/Module.fst",
+    "line": 10, "column": 5,
+    "symbol": "my_function"
+  }}}'
+```
+
+**Returns:** `kind` ("symbol"/"module"/"not_found"), `name`, `type_info`, `defined_at`.
+
+### 4. Other Tools
+
+- **`list_sessions`**: List all active sessions with their file paths.
+- **`restart_solver`**: Restart Z3 for a session (useful when solver gets stuck).
+- **`get_proof_context`**: Get proof obligations from tactic-based proofs.
+- **`update_buffer`**: Add/update files in F*'s virtual file system (vfs-add) for dependency resolution.
+- **`close_session`**: Clean up a session.
+
+## HaclPulse Configuration
+
+For verifying HaclPulse code, use these include dirs and options:
+
+```json
+{
+  "file_path": "/path/to/HaclPulse/code/sha3/Module.fst",
+  "cwd": "/path/to/HaclPulse",
+  "include_dirs": [
+    "lib", "specs",
+    "code/sha3", "code/chacha20", "code/salsa20", "code/sha2",
+    "code/blake2", "code/curve25519", "code/poly1305",
+    "obj",
+    "../pulse/out/lib/pulse", "../pulse/out/lib/pulse/lib",
+    "../karamel/krmllib", "../karamel/krmllib/obj"
+  ],
+  "options": [
+    "--cache_dir", "obj",
+    "--already_cached", "Prims FStar LowStar C Spec.Loops TestLib WasmSupport PulseCore Pulse.Lib Pulse.Class Pulse.Main",
+    "--ext", "pulse:rvalues",
+    "--ext", "fly_deps",
+    "--ext", "optimize_let_vc",
+    "--warn_error", "@240+241@247-272-274-288@319@328@331@332@337"
+  ]
+}
+```
+
+## Tips
+
+- **Incremental workflow**: Create session once (slow, loads deps), then `typecheck_buffer`
+  repeatedly (fast, only re-checks changed fragments).
+- **Module name must match file path**: The `module` declaration in `code` must match the
+  file name from `create_session` (e.g., file `Spec.Poly1305.fst` → `module Spec.Poly1305`).
+- **Pulse files**: No special configuration beyond including pulse dirs. The `#lang-pulse`
+  directive in the file is handled automatically.
+- **Lax mode**: Use `"lax": true` for fast syntax/type checking without SMT verification.
+- **Position-based verification**: Use `"kind": "verify-to-position"` with `to_line` to verify
+  only up to a specific point — useful for large files where you're working on one function.
+- **Symbol lookup**: Works after a successful typecheck. Useful for finding types and
+  definition locations of functions you need to call.
+- **Session replacement**: Creating a new session with the same `file_path` replaces the old one.