feat(search): hybrid code search — exact + semantic in one call (#67)

[Doorman11991]

Hybrid code search — exact + semantic in one call

Implements the ideas from #67: a search tool that combines regex/keyword AND semantic search in the same call, over a symbol-aware (AST-ish) local index, fully on-device.

What it does

New hybrid_search tool ("grep on steroids"):

• Exact matching (regex/keyword) — the precision of grep.
• Semantic ranking — surfaces code that does what you describe even when it doesn't contain the query words.
• Symbol-aware chunks — files are split into function/class/method-centered chunks via lightweight definition-boundary detection across JS/TS/Python/Go/Rust/Java/etc., so semantic ranking operates on coherent units.
• One call, fused score — BM25 + a hashed bag-of-words vector, with a boost when a chunk also matches exactly. Exact+semantic hits are marked ●, semantic-only ○.

Modes: hybrid (default), regex, keyword, semantic.

Example (run against this repo):

hybrid_search "restore terminal on suspend"
● src/tui/terminal.js:106 suspend   [score 13.86]
● src/tui/fullscreen.js:758 ...     (Ctrl+Z handler)

None of those lines contain all the query words — semantic ranking finds them.

On the referenced projects

#67 suggested colgrep (Rust + ColBERT multi-vector) and semble (Python + model2vec). Both are great, but they pull in heavy native/Python runtimes and downloaded model weights. SmallCode's whole premise is staying small and fully local with zero external services, so this reuses the existing local hybrid engine (src/rag/index_store) instead of shipping an embedding model — same single-call hybrid ergonomics, no new dependencies, runs instantly on CPU. If someone wants true neural embeddings later, this leaves room to layer a semantic MCP on top.

Changes

• New: src/tools/hybrid_search.js, test/hybrid_search.test.js (11 cases).
• Wired into: bin/executor.js (tool case, path-contained via safeResolvePath), bin/tools.js (schema), src/compiled/tool_router.js + src/tools/two_stage_router.js (search/code-intel categories), src/tools/dedup.js (marked pure).
• Docs: README feature section + CHANGELOG.
• Tunables: SMALLCODE_HYBRID_MAX_FILES (1500), SMALLCODE_HYBRID_MAX_BYTES (512KiB).

Testing

Full suite 313 passing (node --test test/*.test.js, excluding the pre-existing environment-dependent shell_session.test.js). Build clean.

Closes #67.