Phase 1: Wire symbol index to context chain — replace regex-based resolution with SQLite queries

[ajianaz]

Goal

Replace regex-based symbol resolution in build_context_chain() with SQLite symbol index lookups for higher accuracy and speed.

Context

• Current: engine/context/resolver.rs uses regex to extract imports, fn calls, type refs from diff, then resolves via file-system scan (std::fs::read_dir + grep-style matching)
• cora index builds a complete symbol index in SQLite (tree-sitter parsed) that is never queried by review/scan pipeline
• This means cora index data goes unused — symbol resolution is re-done ad-hoc every review

Scope

1. Add resolve_via_index() in engine/context/resolver.rs — query SQLite symbols table instead of regex+fs
2. Add IndexBridge abstraction (engine/index_bridge.rs) — open cora.db if available, graceful degradation if not
3. Add prefer_index: bool to ContextConfig (default: true if index available)
4. Fallback to regex-based resolution when index unavailable

Files

• src/engine/context/resolver.rs — add index-based resolution
• src/engine/context/mod.rs — IndexBridge integration
• src/engine/context/types.rs — prefer_index config
• NEW: src/engine/index_bridge.rs — index availability abstraction

Benefits

• More accurate resolution (tree-sitter parsed vs regex)
• Faster for large repos (SQLite query < file walk)
• No more 400-file scan limit for blast radius

Effort: 3-4 days