## Summary
- Let `close_agent` clean up an agent that is still registered in
`AgentRegistry` even when its underlying thread is already missing.
- Preserve the explicit-close boundary: for known stale thread-spawn
agents, mark the persisted spawn edge `Closed`, then treat
`ThreadNotFound` / `InternalAgentDied` as a successful close so the
registry slot can be released.
- Add a regression for MultiAgentV2 task-name targets where
`close_agent("worker")` succeeds after the worker thread has already
disappeared.
## Motivation
A worker can disappear from `ThreadManager` while its metadata still
exists in the root `AgentRegistry`. Before this change, the close tool
failed while trying to subscribe to the missing thread status, so it
never reached the cleanup path that releases the registered agent slot.
With `agents.max_threads = 1`, an explicit close of that stale task-name
agent could fail and leave the session unable to spawn a replacement.
## Scope
This PR intentionally does not add automatic stale-agent reaping to
`spawn_agent`, `resume_agent`, or `list_agents`. A thread being missing
from `ThreadManager` is not the same as an explicit close: persisted
open spawn edges are still the durable source of truth for resume and
task-name ownership until `close_agent` is called.
## Validation
- `just test -p codex-core -E
'test(multi_agent_v2_close_agent_reaps_stale_task_name_target) |
test(resume_agent_from_rollout_reopens_open_descendants_after_manager_shutdown)'`
- `just fix -p codex-core`
jif-oai
e2551a5e36