feat: auto-prune stale session artifacts on startup + daily

Session files (JSONLs, debug logs, todos, telemetry, group logs) accumulate
unboundedly — especially from daily cron tasks. This adds a cleanup script
that prunes old artifacts while protecting active sessions (read from DB),
and wires it into the main process on a 24h interval.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/index.ts

@@ -61,6 +61,7 @@ import {
   loadSenderAllowlist,
   shouldDropMessage,
 } from './sender-allowlist.js';
+import { startSessionCleanup } from './session-cleanup.js';
 import { startSchedulerLoop } from './task-scheduler.js';
 import { Channel, NewMessage, RegisteredGroup } from './types.js';
 import { logger } from './logger.js';
@@ -746,6 +747,7 @@ async function main(): Promise<void> {
       }
     },
   });
+  startSessionCleanup();
   queue.setProcessMessagesFn(processGroupMessages);
   recoverPendingMessages();
   startMessageLoop().catch((err) => {