feat: add /stop command to kill running agent process

- /stop, /cancel, /kill aliases all supported
- SIGTERM first (allows graceful AbortController cleanup), SIGKILL after 5s
- Wired through session-commands → GroupQueue.killProcess()

src/session-commands.test.ts

@@ -176,6 +176,7 @@ function makeDeps(
     isAdminSender: vi.fn().mockReturnValue(false),
     canSenderInteract: vi.fn().mockReturnValue(true),
     markReviewReady: vi.fn().mockResolvedValue('Review snapshot updated.'),
+    killProcess: vi.fn().mockReturnValue(false),
     ...overrides,
   };
 }