feat: implement graceful agent shutdown and update dev script

- Add graceful shutdown to AgentManager that waits for processing agents to complete
- Update server to handle SIGTERM and SIGINT with proper agent cleanup
- Switch dev script to use node --watch --import tsx for faster restarts
- Add shutdown timeout handling with configurable wait period

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/server/package.json

@@ -4,7 +4,8 @@
   "description": "Voice assistant backend server",
   "type": "module",
   "scripts": {
-    "dev": "NODE_ENV=development tsx watch --ignore '**/*.timestamp-*' src/server/index.ts",
+    "dev": "NODE_ENV=development node --watch --import tsx src/server/index.ts",
+    "dev:tsx": "NODE_ENV=development tsx watch --ignore '**/*.timestamp-*' src/server/index.ts",
     "build": "tsc -p tsconfig.server.json",
     "start": "NODE_ENV=production node dist/server/index.js",
     "typecheck": "tsc -p tsconfig.server.json --noEmit",