feat: call graph queries

[Lucas-Bur]

What to build

Add call graph query capability: \pix query "who calls authenticate?" --mode call-graph\ returns callers and callees of a given function. This requires building a call/import/type graph at index time (likely via tree-sitter AST analysis) and storing it alongside the vector index.

Support two query modes:

• --mode callers\ — what functions call X?
• --mode callees\ — what does X call?

This is HITL because it requires significant architectural decisions: graph storage format, tree-sitter call resolution strategy, cross-file import tracking, and how to handle dynamic/indirect calls.

Acceptance criteria

• Call graph built at index time via tree-sitter AST analysis
• \pix query "authenticate" --mode callers\ returns all caller functions
• \pix query "authenticate" --mode callees\ returns all functions called by authenticate
• Cross-file calls resolved via import analysis
• Graph stored efficiently alongside existing index files
• Works with incremental indexing (graph updates only for changed files)
• Decision on graph storage format documented (adjacency list, edge list, etc.)

Blocked by

• feat: AST-aware chunking with tree-sitter #83 (AST-aware chunking with tree-sitter)
• feat: incremental indexing via mtime cache #84 (Incremental indexing via mtime cache)

[Lucas-Bur]

Context from design session

This issue was approved as a later-stage feature. Key decisions from our discussion:

• Deferred until after feat: AST-aware chunking with tree-sitter #83 and feat: incremental indexing via mtime cache #84: Call graph queries require AST-aware chunking (for symbol extraction) and incremental indexing (for graph updates on changed files).
• Not a retrieval path in the RRF sense: This is a separate query mode (--mode callers / --mode callees), not a parallel retrieval path that feeds into RRF fusion.
• Graph storage: Needs architectural decision on format (adjacency list, edge list, etc.) — deferred until implementation.
• Cross-file calls: Must resolve via import analysis — non-trivial for dynamic languages.

Future Graph Features (not in scope)

• Type graph retriever (find by type signature, interface implementations)
• Data flow retriever (trace variable definitions to uses)
• Inheritance hierarchy search (find subclasses, overrides)
• GNN on code graphs (encode AST/CFG as GNN embeddings)

This is part of the "specialized retrievers" category in our feature roadmap.