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/multi-agent-mux-delegate-job/multi-agent-mux-delegate-job

@@ -855,7 +855,7 @@ _registry_wait() {
   local job_id="$1" sess="$2" res
   _decided res wait-open-jobs --job "$job_id" --agent-session "$sess" --registry-dir "$REGISTRY_DIR" \
       --wait-s "$(mam_int_knob MAM_DISPATCH_WAIT_S 600 0)" ${START_TIME:+--start-time "$START_TIME"} \
-      --timeout "$TIMEOUT" --watch-pid "$$"
+      --timeout "$TIMEOUT" --watch-pid "$$" --hold
   case "$res" in
     clear) return 0 ;;
     busy*)