feat: add search_code tool wrapping the cs code-search engine

Add a dedicated, permission-gated search_code tool that wraps boyter/cs
(code spelunker) — a fast, relevance-ranked, structure-aware code search
engine — giving agents a better default than grep/find for exploratory
'where is X / how does Y work' searches (ranked results, snippets, ~5x
smaller payloads).

- packages/core/src/tools/search-code.ts: createSearchCodeTool factory;
  -f json invocation, workdir path containment, graceful missing-binary
  handling (DISPATCH_CS_BIN override), readable per-file formatted output.
- Wire-up: export from core; register in agent-manager (both child-whitelist
  and parent perm paths) behind new perm_search_code; add to summon catalog
  + tools enum; frontend ToolPermissions + settings.
- Docker: build a patched, statically-linked cs (pinned v3.1.0 commit) in a
  golang builder stage and bundle at /usr/local/bin/cs.
- docker/cs/luau-declarations.patch: additive Luau declaration table so
  --only-declarations / definition ranking works for Roblox .luau files
  (upstream has Lua but not Luau). Applied during the Docker build.
- Tests: new search-code.test.ts (stubbed JSON formatting + live-cs
  integration, skipped when cs absent); agent-manager/routes mocks +
  perm-gating assertions; loader pass-through.

All tests (596), biome, and tsc (core/api/frontend) pass. cs-builder Docker
stage verified to build and produce a working patched binary.

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