fix(issue-50): the registry wait admits one job per session and holds it (review aa1bff92 F1, F2)

F1: `clear` is now an admission persisted on the released record - a merge
through the engine whose new `guard` re-reads the session's open jobs under
the registry lock, so two waiters cannot both be admitted. It is scoped to
the attempt by the dispatch token (a redispatch voids it) and reported as
`admitted` to every other waiter, which holds the session as a running job
does, whatever its FIFO place: a lower id in the same second or a
redispatched older loop job now waits.
F2: a pending record's liveness window is the dispatcher's signal cadence
times three misses (10 s x 3 = 30 s), no longer ack_timeout_sec, which the
agent's budget takes down to 1 s. After `clear` a detached hold renews the
admitted record's signal until the record leaves pending, its attempt
changes, the wrapper is gone, or the wall clock ends.
Tests: u1 boundaries, u1d/u2e/u2f/u2g/u3d/u3e for the admission, u4c/u4d for
the hold, s1 pins --hold; real herdr: C3/C7/C8 adjusted, C9 is C8's
live-owner counterpart with short ack windows.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.agents/skills/lib_py/delegation/engine.py

@@ -284,11 +284,16 @@ def _notification(job_id, kind, **fields) -> Dict[str, Any]:
 def transition(job_id, request="merge", *, registry_dir=None, expected_status=None,
                expected_attempt=None, fields=None, source="unknown",
                notify=None, expected_notification=_UNSET,
-               expected_dispatch_token=None) -> TransitionResult:
+               expected_dispatch_token=None, guard=None) -> TransitionResult:
     """Decide and commit one state transition. The only writer of job status.
 
     Returns a result whose `notifications` have ALREADY been flushed; the field
     is kept so callers and tests can see what the transition emitted.
+
+    `guard(record)`, when given, runs under the registry lock before the
+    decision and refuses the commit by returning false: a precondition that
+    spans other records (Issue #50 dispatch admission), which the per-record
+    expectations cannot express. A refusal by the guard is not a conflict.
     """
     registry_dir = registry_dir or mqtt_common.DEFAULT_REGISTRY_DIR
     fields = dict(fields or {})
@@ -297,10 +302,13 @@ def transition(job_id, request="merge", *, registry_dir=None, expected_status=No
     with mqtt_common.registry_lock(registry_dir):
         record = mqtt_common.load_job(job_id, registry_dir)
         from_status = record.get("status")
-        decision = _decide(record, request, expected_status, expected_attempt,
-                           source=source, fields=fields,
-                           expected_notification=expected_notification,
-                           expected_dispatch_token=expected_dispatch_token)
+        if guard is not None and not guard(record):
+            decision = _Decision(False, from_status, "guard refused")
+        else:
+            decision = _decide(record, request, expected_status, expected_attempt,
+                               source=source, fields=fields,
+                               expected_notification=expected_notification,
+                               expected_dispatch_token=expected_dispatch_token)
 
         notifications: List[Dict[str, Any]] = []
         if decision.commit: