feat(extraction): content-based generated-file detection (CG-5, #1500)

`isGeneratedFile` was path-only, but Go's own convention is a CONTENT
marker (`// Code generated by <tool>. DO NOT EDIT.`), not a filename one.
A Go monorepo with generated CRUD in ordinarily-named files sitting beside
hand-written use-cases was therefore invisible to every generated-file
down-rank in the codebase — that is #1500.

Measured on kubernetes/client-go (2,453 Go files): the canonical banner
appears in 2,001 of them, the path check flags 0, the new content check
flags exactly those 2,001 — no false positives, no misses.

Design: decide at INDEX time (content is already in memory for parsing),
persist on `files.generated`, read from the DB. Explore never reads file
headers per request.

- `hasGeneratedHeader(content)` recognizes the standard banners — Go's,
  protoc's, `@generated`, `<auto-generated>`, Thrift, OpenAPI Generator,
  FlatBuffers, bindgen, ANTLR. Precision-first and fenced three ways: an
  8KB/60-line header window, a comment-line requirement (leader or open
  block comment), and markers tight enough that prose can't trip them. A
  generator's own source, holding the banner as a string constant in its
  body, is not flagged; neither is this module itself (pinned by test).
- `isGeneratedFile(path)` is unchanged — cheap, sync, still the fallback.
- Schema v9 adds `files.generated` + a PARTIAL index. DDL only, no
  backfill: the flag derives from content the migration cannot see, so
  rows stay 0 until a re-index and every reader unions the flag with the
  path check — an un-migrated index keeps pre-#1500 behavior rather than
  regressing. Re-index required; noted in the CHANGELOG.
- `generatedPredicateFor(paths)` gives ranking a bounded probe + O(1)
  lookups. Bounded, not cached: no invalidation, so a ranking call can
  never serve a verdict the last sync already replaced. Wired into explore
  ranking, findSymbolMatches, findAllSymbols, search (MCP + CLI), the
  context formatter, and the dominant-file/route-file hygiene filters.

Cost (acceptance bar was no measurable index-time regression): a single
unanchored `/generat/i` test over the header rejects ~every hand-written
file before any line splitting. 4.6 µs/file on client-go (worst case —
82% generated). End-to-end `codegraph init` on client-go, n=3 alternating
arms: 5.73s median with detection vs 5.76s path-only baseline; the arms
cross over between runs, so the difference is inside run-to-run noise.

Scope note: generated status remains a stable TIEBREAK at equal score,
exactly where it was. Making it a strong negative signal is CG-10, which
this unblocks by making the signal correct and available.

Two pre-existing tests hard-coded schema version 8; both now track
CURRENT_SCHEMA_VERSION (or the migration table) so future migrations
don't require editing them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

__tests__/pr19-improvements.test.ts

@@ -298,8 +298,21 @@ describe('Best-Candidate Resolution', () => {
 
 describe('Schema v2 Migration', () => {
   it.skipIf(!HAS_SQLITE)('should have correct current schema version', async () => {
-    const { CURRENT_SCHEMA_VERSION } = await import('../src/db/migrations');
-    expect(CURRENT_SCHEMA_VERSION).toBe(8);
+    const { CURRENT_SCHEMA_VERSION, getPendingMigrations } = await import('../src/db/migrations');
+    const { DatabaseConnection } = await import('../src/db');
+
+    // The constant must track the migration table, not a literal — a literal
+    // just makes every schema change edit this test (v9/#1500 was the latest).
+    // A fresh database records the current version, so nothing is pending;
+    // ask a version-0 database instead to see the full migration list.
+    const dbPath = path.join(createTempDir(), 'schema-version.db');
+    const conn = DatabaseConnection.initialize(dbPath);
+    const raw = conn.getDb();
+    raw.prepare('DELETE FROM schema_versions').run();
+    const highest = Math.max(...getPendingMigrations(raw).map((m) => m.version));
+    conn.close();
+
+    expect(CURRENT_SCHEMA_VERSION).toBe(highest);
   });
 
   it.skipIf(!HAS_SQLITE)('should have migration for version 2', async () => {