codedb_remote: add 'wiki' backend (wiki.codes) alongside codegraff

[justrach]

Resolves #311.

Summary

`codedb_remote` today hits `codedb.codegraff.com` only. Its sibling project codedb-cloud / wiki.codes is a Zig-native parquet router on Hetzner with more repos indexed, the exact-identifier-lookup `symbol` action, and hot-pin `policy` — but agents using codedb have no path to it.

This PR adds an optional `backend` field. Default stays `"codegraff"` — every existing caller is unchanged on the wire. Opt in with `backend="wiki"` to reach the new capabilities.

Surface

action	codegraff (default)	wiki
tree	✓	✓
outline	✓	✓
search	✓	✓
meta	✓	✗
symbol	✗	✓ (new)
policy	✗	✓ (new)

Cross-backend mismatches get a clear error:
```
action 'meta' not supported on backend 'wiki'
(wiki supports: tree, outline, search, symbol, policy)
```

Wire details

• URL: `https://www.wiki.codes/api/query?slug=&endpoint=&q=` via the Vercel proxy. Server-side-auths to the Hetzner router. No client secrets needed.
• Slug: derived by replacing `/` with `-` (matches wiki's canonical naming: `rust-lang/rust` → `rust-lang-rust`, `vercel/next.js` → `vercel-next-js`).
• Auth: none from the agent's side — same UX as the existing codegraff backend.
• Validation: same repo-format + path-traversal rules as today.

Verified live

```
$ echo '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"codedb_remote",
"arguments":{"repo":"rust-lang/rust","action":"symbol","query":"HashMap",
"backend":"wiki"}},"id":2}' | codedb mcp

→ 25 hits in 686ms, including:
library/std/src/collections/hash/map.rs:247 struct_def "pub struct HashMap<"
library/std/src/collections/hash/map.rs:256 impl_block
src/tools/clippy/tests/ui-toml/toml_disallowed_types/.../conf_disallowed_types.rs:12
src/tools/rust-analyzer/crates/ide-assists/src/handlers/auto_import.rs:408
...
```

Also verified negative paths:

• `backend=wiki action=meta` → clear rejection
• `backend=codegraff action=symbol` → clear rejection
• `backend=garbage` → clear rejection

Out of scope

• Auto-failover between backends (codegraff 404 → try wiki). Follow-up if agents want a single "best effort" mode.
• Writing to wiki (`/admin/ingest`). That's authenticated; bundling it would mean exposing a token. Read-only for now.
• Swapping the default backend. Every existing caller stays on codegraff unless they opt in.

Test plan

• `zig build` green
• Live `codedb_remote` call with `backend=wiki` against rust-lang/rust returns 25 real HashMap definitions
• Validation rejects cross-backend action mismatches with useful error text
• Existing codegraff default behavior unchanged (same URL, same result shape)

🤖 Generated with Claude Code

[github-actions]

Benchmark Regression Report

Threshold: 10.00%

Tool	Base (ns)	Head (ns)	Delta	Status
codedb_bundle	313698	318584	+1.56%	OK
codedb_changes	33458	31214	-6.71%	OK
codedb_deps	4771	4918	+3.08%	OK
codedb_edit	4414	4447	+0.75%	OK
codedb_find	44865	43519	-3.00%	OK
codedb_hot	54465	61655	+13.20%	FAIL
codedb_outline	150882	156435	+3.68%	OK
codedb_read	54158	55494	+2.47%	OK
codedb_search	142237	144327	+1.47%	OK
codedb_snapshot	2024954	2050554	+1.26%	OK
codedb_status	242217	240172	-0.84%	OK
codedb_symbol	35977	43571	+21.11%	FAIL
codedb_tree	45324	47138	+4.00%	OK
codedb_word	39926	43020	+7.75%	OK

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve unambiguous repo identity in wiki slug mapping

The wiki path derivation flattens owner/repo by replacing only / with -, which makes distinct valid repos collide (for example, foo/bar-baz and foo-bar/baz both map to foo-bar-baz). When both are indexed upstream, codedb_remote can return results for the wrong repository with no error, which can silently corrupt analysis. Use an unambiguous encoding (e.g., separate owner/repo params or escaping component dashes) before building slug.

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