fix: persist session completion to database in completeByDbId (#1532)

completeByDbId only cleaned up in-memory state, leaving sdk_sessions rows
with status='active' and completed_at=NULL indefinitely. Ghost sessions
accumulated and exhausted the agent pool, causing 60s timeout errors.

- Add SessionStore.markSessionCompleted() to set status/completed_at/completed_at_epoch
- Call it at the start of completeByDbId before in-memory cleanup
- Inject SessionStore into SessionCompletionHandler via constructor
- Add 4 tests covering status, timestamps, isolation, and non-existent IDs

Closes #1532

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

src/services/sqlite/SessionStore.ts

@@ -886,6 +886,16 @@ export class SessionStore {
     `).run(memorySessionId, sessionDbId);
   }
 
+  markSessionCompleted(sessionDbId: number): void {
+    const nowEpoch = Date.now();
+    const nowIso = new Date(nowEpoch).toISOString();
+    this.db.prepare(`
+      UPDATE sdk_sessions
+      SET status = 'completed', completed_at = ?, completed_at_epoch = ?
+      WHERE id = ?
+    `).run(nowIso, nowEpoch, sessionDbId);
+  }
+
   /**
    * Ensures memory_session_id is registered in sdk_sessions before FK-constrained INSERT.
    * This fixes Issue #846 where observations fail after worker restart because the