fix: coordinate sqlite index writers

Implement the full ADR-0006 plan: three-layer separation of transaction
correctness, retry policy, and cross-process writer coordination.

Layer 1 — scripts/tx.ts (transaction correctness):
- runWriteTransaction executes work exactly once; no internal retry.
- BEGIN IMMEDIATE takes the write lock up front (avoids SQLITE_BUSY_SNAPSHOT).
- Guarded rollback: checks inTransaction() via adapter before attempting
  ROLLBACK; never masks the primary exception.
- WriteTxDiagnostics attached to errors: phase, code, label,
  rollbackSucceeded, rollbackError, transactionActive.
- Binding adapters (betterSqliteTransactionAdapter, nodeSqliteTransactionAdapter)
  mapping better-sqlite3's `.inTransaction` and node:sqlite's `.isTransaction`.
- configureConnection centralizes WAL + synchronous + busy_timeout.

Layer 2 — scripts/write-coordinator.ts (retry policy):
- runRetryableWriteTransaction: bounded retry with total time budget.
- Only retries when the transaction confirmed ended (transactionActive=false)
  and the error is SQLITE_BUSY during work/commit phase.
- BEGIN-phase BUSY = abort entire build (isBeginBusyFailure); the caller
  returns `{ deferred: true, reason: 'writer_busy' }` instead of waiting.
- hasUnusableTransaction detects a still-active transaction after failure;
  aborts the build immediately, never retries.

Layer 3 — scripts/writer-lease.ts (cross-process coordination):
- acquireWriterLease: dedicated writer.lock.sqlite with busy_timeout=0 +
  BEGIN IMMEDIATE. Non-blocking attempt; bounded wait with retryDelayMs.
- writerLockPathFor derives lock path from the target DB path.
- Lease held for the entire build; released on completion or failure.
- Lock DB uses DELETE journal (not WAL); crash/close auto-releases.
- All consumers obey: skill acquires at build start (returns deferred if
  unavailable); app daemon (via worker) acquires for its build cycle.

Build semantics changes:
- affectedSessionIds updated only after successful commit.
- BuildIndexResult gains skipped/skippedFiles for observability.
- Skill finalize failure now fails the build (was silently warned).
- Checkpoint changed to PASSIVE (TRUNCATE reserved for maintenance/exit).
- Skill buildIndex returns { deferred, reason } on lease contention;
  indexer-service reschedules the build (deferredRetryMs) without publishing
  a heartbeat (so the build-deferred state is visible to cross-process
  arbitration).
- Service publishes heartbeat immediately on start() for correct arbitration.

Tests:
- tests/write-transaction.test.mjs: single-shot execution, diagnostics
  propagation, auto-rolled-back transaction detected, rollback failure
  captured as metadata, BEGIN IMMEDIATE semantics.
- tests/writer-lease.test.mjs: acquire/release, contention returns null,
  bounded wait with release during budget.
- tests/app-writer-lease.test.mjs: better-sqlite3 adapter integration.
- tests/app-rollback-guard.test.mjs: rewritten — transient BUSY recovered
  by coordinator, persistent BUSY skips file, begin-busy aborts build,
  live-transaction aborts build, phantom affectedSessionIds prevented.
- tests/daemon-arbitration.test.mjs: skill defers to fresh app heartbeat,
  builds when heartbeat is stale.
- tests/app-indexer-service.test.mjs: new cases for deferred-retry
  scheduling and immediate heartbeat on start.
- app/tests/electron-concurrency.mjs + child: dual-child IPC structure for
  real better-sqlite3 contention (holder acquires lock → build child starts
  → delayed release → result collected; persistent contention bounded).

ADR-0006 updated to reflect the implemented design.

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

tests/app-indexer.test.mjs

@@ -31,7 +31,7 @@ class TestDatabase {
   }
 }
 
-test('app indexer builds the Obelisk database from Claude JSONL and records app heartbeat', () => {
+test('app indexer records build success without claiming daemon ownership', () => {
   const home = mkdtempSync(join(tmpdir(), 'obelisk-app-indexer-'));
   const claudeDir = join(home, '.claude');
   const projectDir = join(claudeDir, 'projects', '-tmp-obelisk-app');
@@ -57,7 +57,7 @@ test('app indexer builds the Obelisk database from Claude JSONL and records app
   assert.deepEqual(firstBuild.affectedSessionIds, [sessionId]);
   assert.equal(firstBuild.ftsRebuilt, true);
   assert.equal(db.prepare("SELECT uuid FROM messages_fts WHERE messages_fts MATCH 'hello'").get().uuid, 'msg-app-1');
-  assert.equal(db.prepare("SELECT jsonl_path FROM index_state WHERE jsonl_path='__app_heartbeat__'").get().jsonl_path, '__app_heartbeat__');
+  assert.equal(db.prepare("SELECT jsonl_path FROM index_state WHERE jsonl_path='__app_heartbeat__'").get(), undefined);
   assert.equal(db.prepare("SELECT jsonl_path FROM index_state WHERE jsonl_path='__app_last_successful_build__'").get().jsonl_path, '__app_last_successful_build__');
   assert.equal(db.prepare('SELECT project_path FROM sessions WHERE id=?').get(sessionId).project_path, '/tmp/obelisk-app');
   db.close();