Previously crash recovery recreated detached sessions as if they were
fully managed, so polls and kills could lie about liveness and the
checkpoint could forget recovered jobs after the next restart.
This commit refreshes recovered host-backed sessions from real PID
state, keeps checkpoint data durable, and preserves notify watcher
metadata while treating sandbox-only PIDs as non-recoverable.
- Persist `pid_scope` in `tools/process_registry.py` and skip
recovering sandbox-backed entries without a host-visible PID handle
- Refresh detached sessions on access so `get`/`poll`/`wait` and active
session queries observe exited processes instead of hanging forever
- Allow recovered host PIDs to be terminated honestly and requeue
`notify_on_complete` watchers during checkpoint recovery
- Add regression tests for durable checkpoints, detached exit/kill
behavior, sandbox skip logic, and recovered notify watchers
* 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
