feat(kg): add fixed code graph query verbs

[edheltzel]

Closes #205

Summary

• add code graph traversal library for callers, callees, and bounded trace over code_edges
• expose recall callers, recall callees, and recall trace fixed verbs
• add friendly code graph capability guards for old/missing schema on index/query verbs
• cover resolved, unresolved, trace, and guard behavior in command tests

Verification

• bun test tests/commands/code-graph.test.ts tests/lib/code-indexer.test.ts
• RECALL_DB_PATH=.tmp-recall-kg-205.db bun src/index.ts index src --project Recall
• RECALL_DB_PATH=.tmp-recall-kg-205.db bun src/index.ts callers indexCode -p Recall
• RECALL_DB_PATH=.tmp-recall-kg-205.db bun src/index.ts callees runIndexCode -p Recall
• RECALL_DB_PATH=.tmp-recall-kg-205.db bun src/index.ts trace runIndexCode indexCode -p Recall
• codegraph callers indexCode
• codegraph callees runIndexCode
• bun run lint
• bun run build
• bun test