feat(tool): search_code — dedup + rank find_code matches (Issue #10)

[kellenff]

Summary

Implements issue #10, part 1: a new `search_code` MCP tool that wraps `find_code` matches into their containing functions, dedupes by NodeID, and ranks by structural importance.

Semantic search (part 2) deferred per the YAGNI note in the issue — embeddings cost disk, RAM, and a build dep; ship dedup+rank first, add when a real consumer asks.

What it does

Same args as `find_code` (pattern, pattern_kind, scope, file_filter, limit). Output is an array of `groups`:

```
{
groups: [
{
nodeId, kind, summary, file, enclosingRange,
matchCount, matches: [{line, snippet}],
bucket: "definition" | "popular" | "test"
}
],
provenance: { tool: "yactt", version, fetchedAt }
}
```

Ranking: bucket tier → matchCount desc → fanIn desc → nodeId asc, stable.

Files

• new `internal/tool/searchcode.go` — handler factory, args, schemas, group + rank logic. ~190 lines.
• new `internal/tool/searchcode_test.go` — dedup, bucketing, tree-sitter path, limit cap, provenance, regex validation, length cap, stable sort.
• new `tests/acceptance/searchcode_test.go` — dedup against fixture (3 `err` refs in `Login` collapse to one group), test-bucket assertion (`login_test.go` references land in `test`), provenance stamp.
• mod `cmd/yactt/main.go` — registers `search_code` in both the `tools` slice and the `persisted_query` `toolFuncs` map.
• mod `internal/tool/wire_shape_test.go` — adds the contract-test row.
• mod `README.md` + `plugins/yactt/README.md` — bumps tool counts (19→20 / 10→11), adds the `search_code` row + mention.

Reuse

Heavy lifting already lives in `internal/tool/findcode.go`:

• `findCodeRegex(repo, scope, fileFilter, rx, true, limit)` — regex scan with enclosing-symbol context per match
• `findCodeTreeSitter(repo, scope, fileFilter, pattern, true, limit)` — tree-sitter scan with same context
• `enclosingContext(repo, path, line)` — already wired to the in-memory symbol index

`search_code` calls these with `withContext=true` and adds only the group + rank layer on top. No re-parsing, no duplicated logic.

Verification

• `go vet ./...` clean
• `go test ./...` green (full suite, ~6 min)
• New unit tests: 8 cases
• New acceptance tests: 3 cases (dedup, test-bucket, provenance)
• `wire_shape_test` exercises the new tool via the JSON-RPC server

Closes #10 (part 1).

🤖 Generated with Claude Code