feat: add watch_patterns to background processes for output monitoring

Adds a new 'watch_patterns' parameter to terminal(background=true) that
lets the agent specify strings to watch for in process output. When a
matching line appears, a notification is queued and injected as a
synthetic message — triggering a new agent turn, similar to
notify_on_complete but mid-process.

Implementation:
- ProcessSession gets watch_patterns field + rate-limit state
- _check_watch_patterns() in ProcessRegistry scans new output chunks
  from all three reader threads (local, PTY, env-poller)
- Rate limited: max 8 notifications per 10s window
- Sustained overload (45s) permanently disables watching for that process
- watch_queue alongside completion_queue, same consumption pattern
- CLI drains watch_queue in both idle loop and post-turn drain
- Gateway drains after agent runs via _inject_watch_notification()
- Checkpoint persistence + crash recovery includes watch_patterns
- Blocked in execute_code sandbox (like other bg params)
- 20 new tests covering matching, rate limiting, overload kill,
  checkpoint persistence, schema, and handler passthrough

Usage:
  terminal(
      command='npm run dev',
      background=true,
      watch_patterns=['ERROR', 'WARN', 'listening on port']
  )

tools/code_execution_tool.py

@@ -301,7 +301,7 @@ def _call(tool_name, args):
 # ---------------------------------------------------------------------------
 
 # Terminal parameters that must not be used from ephemeral sandbox scripts
-_TERMINAL_BLOCKED_PARAMS = {"background", "check_interval", "pty", "notify_on_complete"}
+_TERMINAL_BLOCKED_PARAMS = {"background", "check_interval", "pty", "notify_on_complete", "watch_patterns"}
 
 
 def _rpc_server_loop(