feat(db): phase 1a — native code-KG schema + 15→16 migration

[edheltzel]

Summary

Lands the native code knowledge graph schema as Phase 1a (epic #196). This is the first of three Phase 1 PRs; 1b (indexer command) and 1c (scout integration) build directly on this schema.

Three files changed, all additive — no existing behavior altered.

Changes

src/db/schema.ts

• Appended code_files, code_nodes, code_edges CREATE TABLE IF NOT EXISTS to CREATE_TABLES (fresh-install + existing-install idempotent path via initDb, G1/G7 precedent)
• Added code_nodes entry to FTS_SCHEMA — external-content FTS5 over (name, qualified_name, signature), byte-aligned with the house pattern. Auto-wires recall repair/doctor via FTS_SOURCES with zero extra code (G6)
• Appended 6 indexes to CREATE_INDEXES, including a partial index on unresolved edges (WHERE dst_id IS NULL) that makes the resolution re-link pass cheap

src/db/migrations.ts

• Appended MIGRATIONS[15] — minimal 15→16 capability gate. Body slices FTS DDL from FTS_SCHEMA.code_nodes (G8 no-drift idiom, same pattern as 14→15). Does NOT open its own transaction (runner provides it). Does NOT re-CREATE base tables (initDb's CREATE_TABLES already guarantees them on every init)

tests/db/migrations.test.ts + tests/db/schema.test.ts

• New code-KG schema migration (15 to 16) describe block with 4 tests:
  • Fresh DB lands at version 16 with all 4 objects (3 tables + FTS) — mutation guard
  • v8 fixture migrates cleanly through 16 (per §9-C1 — the live DB is at v8, not v15)
  • code_edges carries no project column (§9-C2)
  • FK semantics: src_id CASCADE, dst_id SET NULL — exercised with real inserts/deletes (§9-C3)
• Updated 4 hardcoded MIGRATIONS.length mutation guards from 15 → 16

§9 RedTeam corrections implemented

Correction	Applied
C1 — test uses sub-15 fixture (v8)	✓ v8 fixture, not v15→16
C2 — code_edges has no project column	✓ omitted; comment explains scope via src_id
C3 — resolved predicate is dst_id IS NOT NULL	✓ DDL comment + FK test use this predicate
C6 — comment describes one try/catch, not per-statement	✓ tightened in migration body

Verification

bun run build and bun test both green. Pre-existing failures in opencode-integration and pi-integration (labeled [RED] — unimplemented hardening tests unrelated to schema/migration) are unchanged.

 1187 pass
 10 fail  ← all 10 are pre-existing [RED] tests in opencode/pi-integration
Ran 1197 tests across 97 files

The new describe block contributes 11 tests (37 total in migrations.test.ts, up from 26).

Closes #199