fix(delegate): submit also takes the target workspace's update admission (Issue #72, review R1)

Decision 8: delegate submit uses the admission lock of the workspace the
target session belongs to. The wrapper resolved only its own
WORKSPACE_ROOT, so a job could be registered and delivered to an agent of
another installed workspace while that workspace was being updated.

lib_py.update target-workspace resolves a herdr:<name> row's workspace (the
.mam at or above its pane.cwd, else its control_plane.workspace_root);
mam_update_admit takes an fd, and submit holds the caller root on fd 7 and
the target on fd 8 until the record is written. Rowless and remote targets
keep the caller-root gate.

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

.agents/skills/lib.sh

@@ -3264,26 +3264,27 @@ mam_control_plane_prefix() {
 # fd 7 (`7>&-` on such a child, as pane_lock's fd 9). The wait is bounded by
 # MAM_UPDATE_ADMISSION_WAIT_S (default 120).
 #
-# mam_update_admit [workspace]: rc 0 admitted (fd 7 held, unless the workspace
-# has no .mam), 1 refused (stderr).
+# mam_update_admit [workspace [fd]]: rc 0 admitted (fd 7, or the given one,
+# held unless the workspace has no .mam), 1 refused (stderr). Delegate submit
+# also takes its target's workspace on fd 8.
 mam_update_admit() {
-  local root="${1:-$WORKSPACE_ROOT}"
+  local root="${1:-$WORKSPACE_ROOT}" fd="${2:-7}"
   local lock="$root/.mam/update-admission.lock" marker="$root/.mam/update-in-progress" st
   # No .mam: not an installed workspace, which is all the update runs in (and a
   # checkout running its own scripts gets no .mam from this).
   [ -d "$root/.mam" ] || return 0
-  if ! exec 7>>"$lock"; then
+  if ! eval "exec $fd>>\"\$lock\""; then
     echo "ERROR: cannot open the update admission lock $lock" >&2
     return 1
   fi
-  case "$(PYTHONPATH="$SKILL_DIR:${PYTHONPATH:-}" python3 -m lib_py.pane_lock acquire --fd 7 --shared --wait-env MAM_UPDATE_ADMISSION_WAIT_S)" in
+  case "$(PYTHONPATH="$SKILL_DIR:${PYTHONPATH:-}" python3 -m lib_py.pane_lock acquire --fd "$fd" --shared --wait-env MAM_UPDATE_ADMISSION_WAIT_S)" in
     locked) ;;
     timeout)
-      exec 7>&-
+      mam_update_admit_release "$fd"
       echo "ERROR: an update of $root holds its admission lock (MAM_UPDATE_ADMISSION_WAIT_S passed); try again when it has finished" >&2
       return 1 ;;
     *)
-      exec 7>&-
+      mam_update_admit_release "$fd"
       echo "ERROR: the update admission lock $lock could not be decided (lib_py.pane_lock)" >&2
       return 1 ;;
   esac
@@ -3296,11 +3297,11 @@ mam_update_admit() {
       echo "NOTE: stale update marker $marker (its update is gone); ignored" >&2
       return 0 ;;
   esac
-  exec 7>&-
+  mam_update_admit_release "$fd"
   echo "ERROR: an update of $root is in progress (marker $marker, holder $st); try again when it has finished" >&2
   return 1
 }
 
 mam_update_admit_release() {
-  exec 7>&-
+  eval "exec ${1:-7}>&-"
 }