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

cli.py

@@ -8134,6 +8134,25 @@ class HermesCLI:
                         # Periodic config watcher — auto-reload MCP on mcp_servers change
                         if not self._agent_running:
                             self._check_config_mcp_changes()
+                            # Check for background process completion notifications
+                            # while the agent is idle (user hasn't typed anything yet).
+                            try:
+                                from tools.process_registry import process_registry
+                                if not process_registry.completion_queue.empty():
+                                    completion = process_registry.completion_queue.get_nowait()
+                                    _exit = completion.get("exit_code", "?")
+                                    _cmd = completion.get("command", "unknown")
+                                    _sid = completion.get("session_id", "unknown")
+                                    _out = completion.get("output", "")
+                                    _synth = (
+                                        f"[SYSTEM: Background process {_sid} completed "
+                                        f"(exit code {_exit}).\n"
+                                        f"Command: {_cmd}\n"
+                                        f"Output:\n{_out}]"
+                                    )
+                                    self._pending_input.put(_synth)
+                            except Exception:
+                                pass
                         continue
                     
                     if not user_input:
@@ -8247,7 +8266,29 @@ class HermesCLI:
                                 except Exception as e:
                                     _cprint(f"{_DIM}Voice auto-restart failed: {e}{_RST}")
                             threading.Thread(target=_restart_recording, daemon=True).start()
-                    
+
+                        # Drain process completion notifications — any background
+                        # process that finished with notify_on_complete while the
+                        # agent was running (or before) gets auto-injected as a
+                        # new user message so the agent can react to it.
+                        try:
+                            from tools.process_registry import process_registry
+                            while not process_registry.completion_queue.empty():
+                                completion = process_registry.completion_queue.get_nowait()
+                                _exit = completion.get("exit_code", "?")
+                                _cmd = completion.get("command", "unknown")
+                                _sid = completion.get("session_id", "unknown")
+                                _out = completion.get("output", "")
+                                _synth = (
+                                    f"[SYSTEM: Background process {_sid} completed "
+                                    f"(exit code {_exit}).\n"
+                                    f"Command: {_cmd}\n"
+                                    f"Output:\n{_out}]"
+                                )
+                                self._pending_input.put(_synth)
+                        except Exception:
+                            pass  # Non-fatal — don't break the main loop
+
                 except Exception as e:
                     print(f"Error: {e}")