feat(issue-30): a stall watcher that only notifies

lib_py/stall_watch.py runs beside the wrapper's subscriber for a local herdr
job (started after delivery, stopped when the subscriber ends; MAM_STALL_WATCH=0
turns it off). It follows `herdr agent wait` transitions and publishes only
`progress` events through one publisher; it has no path that closes a job.

- blocked: `stalled:blocked <detection line>` once per episode, never answered.
- idle for MAM_STALL_IDLE_S (600) with no background descendants of the agent
  pid (pane process-info + /proc; adapter resident_children and the heartbeat
  timer excluded) and an empty input box: `stalled:idle` and one nudge.
- input box judged from adapter facts measured on herdr 0.9.1 (codex '›',
  grok frame '│', opencode '┃' block + stop pattern); the whole-pane fallback
  is gone. No facts: withheld + `stall_watch:input_check_unavailable` once;
  never empty for 3 x S: `stall_watch:input_never_empty` once.
- herdr failures are not "no event": warning + `stall_watch:herdr_unavailable:
  <code>` (or agent_not_found), backoff, rc 2 after MAM_STALL_HERDR_FAIL_S
  (120); the wrapper prints the watcher's last lines.
- remote and human jobs are skipped with one log line.
- agent_status_reliable is an adapter field; false turns idle judgement off.
- the loop runner relays stalled:/stall_watch: notices once each.
- scoped_env drops a pane-inherited workspace scope for name lookups (#34 rule).

The module sits at the top of lib_py: delegation and loop may not import the
agent adapters (their sibling-import rules). Idle pane captures of the five
CLIs are fixtures; the mock herdr answers process-info in the real shape.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fixtures/panes/opencode_idle.txt

@@ -0,0 +1,16 @@
+# captured 2026-09-25 from an idle agent, input box empty: herdr 0.9.1 `herdr agent read reviewer-opencode-01` (last 15 lines)
+     유출시키고 있습니다. 하위 워크스페이스 접두사가 테스트 export와 일치하는 pane에서 실행될 때 H-21이 잘못
+     실패합니다. 이는 제 자신의 w2 pane에서 재현되었으며, 환경을 제거한 상태로 재실행하면 통과됩니다. 한 줄의
+     unset 수정으로 해결됩니다. MAM 자체 사용 모델(pane에서 에이전트 실행 테스트)에서는 이 문제가 재발
+     가능합니다.
+
+     보고서: .mam/jobs/2986037e/opencode-reports/report-final.md — VERDICT: PASS
+
+     ▣  Build · GLM 5.3 Cloud · 26m 12s
+
+  ┃
+  ┃
+  ┃                                                                                                                 ~/PuKi/laa/canary_projects/multi-
+  ┃  Build auto · GLM 5.3 Cloud Ollama                                                                              agent-mux:main
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+   /home/godopu16/PuKi/laa/canary_projects/multi-agent-mux                               629.5K  ctrl+p commands    • OpenCode 1.18.32