feat: prepare DB for shared access (WAL, service partitioning)

Phase 0-2 of data unification:
- Enable WAL mode + busy_timeout for safe concurrent access
- router_state: keys prefixed with SERVICE_ID (lazy migration)
- sessions: composite PK (group_folder, agent_type)
- registered_groups: filtered by SERVICE_AGENT_TYPE on load
- Logger: service name tag for unified log streams

All changes are backward-compatible with the current split setup.
Actual directory merge (Phase 3-4) is a separate step.

src/logger.ts

@@ -1,7 +1,10 @@
 import pino from 'pino';
 
+const serviceName = (process.env.ASSISTANT_NAME || 'claude').toLowerCase();
+
 export const logger = pino({
   level: process.env.LOG_LEVEL || 'info',
+  name: serviceName,
   transport: { target: 'pino-pretty', options: { colorize: true } },
 });