Remove the Stop hook for activity idle tracking

The idle state is now determined by the absence of activity
heartbeats rather than an explicit hook signal, which is more
reliable when the process exits unexpectedly.

src/vm.ts

@@ -220,7 +220,6 @@ async function configureEnvironment(home: string, apiKey: string): Promise<void>
   const hooks = {
     UserPromptSubmit: [{ hooks: [{ type: "command", command: `${activityBin} active` }] }],
     PreToolUse: [{ hooks: [{ type: "command", command: `${activityBin} active` }] }],
-    Stop: [{ hooks: [{ type: "command", command: `${activityBin} idle` }] }],
   }
   const statusLine = { type: "command", command: `/home/${USER}/.local/bin/sandlot-statusline` }
   const settingsJson = JSON.stringify({ apiKeyHelper: "~/.claude/api-key-helper.sh", skipDangerousModePermissionPrompt: true, hooks, statusLine })