Phase 2: Wire code graph to blast radius — replace regex scan with graph query for callers/callees #457

Goal

Replace regex-based blast radius scan (file-system walk + pattern matching, 400 file limit) with code graph queries (find_callers, find_callees).

Context

• Current: resolve_callers() in context chain scans source code with ignore::WalkBuilder + regex per defined symbol (capped at 400 files)
• find_callers() already exists in src/index/graph.rs — reverse call graph traversal via SQLite edges table
• Only used in cora query and MCP server, not wired to review/scan pipeline

Scope

1. Add resolve_callers_via_graph() in engine/context/resolver.rs
2. Query index::graph::find_callers(conn, project_id, name, max) instead of file-system scan
3. If graph unavailable (no index), fall back to existing regex scan
4. Leverage IndexBridge from Phase 1 (Phase 1: Wire symbol index to context chain — replace regex-based resolution with SQLite queries #456)

Files

• src/engine/context/resolver.rs — add graph-based caller resolution
• src/index/graph.rs — existing find_callers (already implemented)

Benefits

• Accurate blast radius (graph has real call edges, no regex misses)
• No 400-file limit (graph query is instant)
• Catches indirect calls via traits/interfaces

Dependencies

• Requires Phase 1: Wire symbol index to context chain — replace regex-based resolution with SQLite queries #456 (IndexBridge abstraction)

Effort: 1-2 days