fix(reset): fence every job registry the reset reads, by its physical directory (#78)

Review round 3 R1 (codex): the registration guard chose the workspace marker
from the path's spelling, so a symlink alias of .mam/jobs or a MAM_JOBS_DIR
registry was not refused. The reset now writes .reset-in-progress into each
registry its open-job check reads (.mam/jobs, MAM_JOBS_DIR, a row's other
control_plane.mam_jobs_dir - the latter only for this workspace's sessions),
each under that registry's lock and before the check, and removes it on exit;
register_job reads the marker inside the registry it writes to. The
precheck's separate lock barrier is subsumed by the fence.

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

.agents/skills/multi-agent-mux-delegate-job/registry.md

@@ -141,8 +141,8 @@ $PY scripts/registry.py status --job <id> --set completed          # set status
 $PY scripts/registry.py pick   --agent-session herdr:claude         # claim → running
 ```
 
-Exit codes: `0` ok, `1` not found / bad status / (`register`) a reset of the
-workspace owning `<ws>/.mam/jobs` is in progress (Issue #78), `3` (`pick`) no
+Exit codes: `0` ok, `1` not found / bad status / (`register`) a reset fenced
+this registry (its `.reset-in-progress` names a live reset, Issue #78), `3` (`pick`) no
 pending job for that session.
 
 ---