feat: notify_on_complete for background processes (#5779)

* feat: notify_on_complete for background processes

When terminal(background=true, notify_on_complete=true), the system
auto-triggers a new agent turn when the process exits — no polling needed.

Changes:
- ProcessSession: add notify_on_complete field
- ProcessRegistry: add completion_queue, populate on _move_to_finished()
- Terminal tool: add notify_on_complete parameter to schema + handler
- CLI: drain completion_queue after agent turn AND during idle loop
- Gateway: enhanced _run_process_watcher injects synthetic MessageEvent
  on completion, triggering a full agent turn
- Checkpoint persistence includes notify_on_complete for crash recovery
- code_execution_tool: block notify_on_complete in sandbox scripts
- 15 new tests covering queue mechanics, checkpoint round-trip, schema

* docs: update terminal tool descriptions for notify_on_complete

- background: remove 'ONLY for servers' language, describe both patterns
  (long-lived processes AND long-running tasks with notify_on_complete)
- notify_on_complete: more prescriptive about when to use it
- TERMINAL_TOOL_DESCRIPTION: remove 'Do NOT use background for builds'
  guidance that contradicted the new feature

tools/code_execution_tool.py

@@ -300,7 +300,7 @@ def _call(tool_name, args):
 # ---------------------------------------------------------------------------
 
 # Terminal parameters that must not be used from ephemeral sandbox scripts
-_TERMINAL_BLOCKED_PARAMS = {"background", "check_interval", "pty"}
+_TERMINAL_BLOCKED_PARAMS = {"background", "check_interval", "pty", "notify_on_complete"}
 
 
 def _rpc_server_loop(