fix(issue-50): review findings of PR #56 on macOS

- admission: a `submit --dry-run` record is marked dry_run and is not an
  open job of its session (review 1e25cf2f F1)
- create: the monitor-row adoption requires the row's server to be the one
  this create spawned on (review 1e25cf2f F2)
- pane lock: retry only on contention (BlockingIOError); the docs say the
  flock lasts while any fd-9 holder (the claim shell and the shim) lives
  (review 66680d9d)
- tests on macOS: real-herdr box HOMEs under /tmp (sun_path is 104 bytes),
  resolved paths, the fake claude reads non-canonical input (MAX_CANON 1024)
  and the pane is read unwrapped; lock holders exec their sleep so killing
  them frees the lock; the adoption tests name the server

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VCgu9Fa9iT7TbJUSzdq4Ff

tests/test_herdr_shim_contract.py

@@ -620,7 +620,7 @@ def test_h26_new_session_claims_its_pane_under_the_servers_lock(mam_sandbox, moc
     locks = glob.glob(str(tmp_path / ".local" / "state" / "mam" / "pane-lock" / "*.lock"))
     assert len(locks) == 1, locks                     # one file per server, under the sandbox HOME
     n_calls = len(_load_mock_state(mock_herdr).get("calls", []))
-    holder = subprocess.Popen(["bash", "-c", 'exec 9>>"$1"; python3 -m lib_py.pane_lock acquire --fd 9; sleep 600',
+    holder = subprocess.Popen(["bash", "-c", 'exec 9>>"$1"; python3 -m lib_py.pane_lock acquire --fd 9; exec sleep 600',
                                "holder", locks[0]], stdout=subprocess.PIPE, text=True,
                               env=dict(os.environ, PYTHONPATH=str(tmp_path / ".agents" / "skills")))
     try: