Structural code tools: find_definition / find_references / symbol_outline / call_graph (tree-sitter)

[imkp1]

Summary

Add structural code-navigation tools (tree-sitter based) alongside the existing semantic search: find_definition, find_references, symbol_outline, and call_graph. Semantic embeddings are fuzzy; structural questions deserve exact answers.

Motivation

The README explicitly advertises "what calls this function?" and "trace a request end to end." Those are structural queries that pure vector search answers only approximately. A symbol/reference index gives precise, verifiable results and is the highest-ROI accuracy improvement to the core loop. It also strengthens citations (#8) and pairs with the lexical fallback direction already in flight (PR #7).

Proposed tools (exposed as indexer MCP tools)

• find_definition(symbol) — locations where a symbol is defined.
• find_references(symbol) — call/usage sites.
• symbol_outline(path) — top-level defs/classes/methods for a file.
• call_graph(symbol) — direct callers/callees (bounded depth).

Implementation notes

• Build a tree-sitter symbol index during the index pass (multi-language via tree-sitter grammars), stored alongside the existing Qdrant data.
• Expose as new indexer MCP tools — the backend injects the indexer MCP endpoint into every ask Job already, and the proxy is transparent, so no backend changes are required.
• Respect the same per-user repo gating as semantic_search (X-User-Token / X-Repo-Access enforced in the indexer's access.py).
• Sequence with PR Make semantic search optional with a graceful lexical fallback #7 (graceful degraded / lexical fallback) — both extend the indexer's non-embedding retrieval surface.

Scope

In-scope: single-workspace, Claude-only. Extends the indexer; no new services.

Acceptance

• The four tools return correct results on the indexed repo for at least Go/Python/TypeScript.
• Tools honour per-user repo access gating.
• Tests added (make test-indexer).