fix(indexer): guard cleanup rollback so it never masks the real build error

Reported as "cannot rollback - no transaction is active" — a secondary error.
SQLite auto-rolls back certain failures (SQLITE_BUSY / SQLITE_BUSY_SNAPSHOT,
disk full), after which the per-file loop's unguarded ROLLBACK in its catch
threw over the real error and aborted the whole build instead of skipping just
the bad file.

Stopgap only (see docs/adr/0006 for the planned full fix):
- Add safeRollback(db) in both indexers: it swallows only the rollback's own
  error, so the true cause surfaces. Per-file failures are logged and the build
  continues; the finalize failure still propagates.
- Give the skill's node:sqlite connection an explicit PRAGMA busy_timeout=5000
  (it has no default). The app adds none: better-sqlite3 already defaults to
  5000ms, so busy_timeout is NOT the root-cause fix and is not treated as one.

Add tests/app-rollback-guard.test.mjs: injects a DB that faithfully reproduces
"a write auto-rolls back the txn, then ROLLBACK errors" and asserts the build
survives (bad file skipped, other file indexed). Revert-checked: without the
guard the test fails with the exact "cannot rollback - no transaction is active".

docs/adr/0006 records the real fix (shared runWriteTransaction, single-writer
coordination, BEGIN IMMEDIATE, whole-transaction retry, PASSIVE checkpointing)
as deferred, two-phase work — and why bumping busy_timeout is not it.

Verified: suite 124/124, typecheck clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/indexer.ts

@@ -35,6 +35,15 @@ interface FileInfo {
   source?: string;
 }
 
+// A rollback inside a catch must never throw over the real error. SQLite
+// auto-rolls back certain failures (SQLITE_BUSY, disk full, ...); a following
+// explicit ROLLBACK then throws "cannot rollback - no transaction is active",
+// which would both mask the true cause and turn a skippable per-file error into
+// a whole-build failure. Swallow only the rollback's own error.
+function safeRollback(db: { exec: (sql: string) => unknown }) {
+  try { db.exec('ROLLBACK'); } catch { /* no active transaction */ }
+}
+
 function resolveSchemaPath() {
   const candidates = [
     path.join(__dirname, 'schema.sql'),
@@ -576,7 +585,7 @@ function buildIndex({
         if (file.source !== 'codex') indexSubagentMeta(db, file);
         db.exec('COMMIT');
       } catch (error) {
-        db.exec('ROLLBACK');
+        safeRollback(db);
         console.warn(`Warning: failed to index ${file.path}: ${(error as Error).message}`);
       }
     }
@@ -596,7 +605,7 @@ function buildIndex({
       if (latestSourceMtime) writeIndexMarker(db, '__last_source_mtime__', latestSourceMtime);
       db.exec('COMMIT');
     } catch (error) {
-      db.exec('ROLLBACK');
+      safeRollback(db);
       throw error;
     }
     return { files: files.length, latestSourceMtime, affectedSessionIds: [...affectedSessionIds], ftsRebuilt };