Wire up `store_kg_edges()` — populate `edges` table

[ajianaz]

Blocking #426.

Problem

edges table exists in schema with columns (source, kind, target, file, line, project_id) and store_kg_edges() is defined in src/index/graph.rs:64 — but it is never called. The table is always empty.

Only call_graph (caller → callee only) is populated. Edge kinds IMPORTS, IMPLEMENTS, INHERITS, CHILD_OF are defined in EdgeKind enum but never extracted or stored.

Task

1. Call store_kg_edges() during indexing in src/index/extract.rs or src/index/mod.rs
2. Extract IMPORTS edges (use/import/require statements per language)
3. Extract IMPLEMENTS edges (impl Trait for Type)
4. Extract INHERITS edges (class extends, struct inheritance)
5. Keep call_graph as-is for backward compat — edges is the unified table
6. Add migration to copy existing call_graph rows into edges as CALLS kind

Files to modify

• src/index/extract.rs — add edge extraction per language
• src/index/mod.rs — call store_kg_edges() after extraction
• src/index/graph.rs — already has store_kg_edges(), verify it works
• src/index/schema.rs — add migration to backfill edges from call_graph

Acceptance

• edges table populated after cora index --rebuild
• SELECT kind, COUNT(*) FROM edges GROUP BY kind returns CALLS + IMPORTS + IMPLEMENTS
• Dead code detection works on edges table (not just call_graph)
• No regression in existing find_callers, impact_analysis, trace_path

Est: 1-2 days

[ajianaz]

Implemented. Closing as completed — feature verified in codebase.