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/grok_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-grok-01` (last 15 lines)
+
+     Worked for 2m54s
+                                                                           █
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