feat(registry): format version 2, migrated by the writer (Issue #71)

- lib_py/registry_format.py: a pure, idempotent migrate() drops
  kill_command/start_command and rebuilds attach_command on every row
  (stopped/terminated included) and sets the top-level registry_format key.
- atomic_dump_yaml migrates an older registry under its lock, before the
  caller's mutation, and then rewrites the YAML as well as the DB (the YAML
  write is otherwise skipped when only fields change). load_state_json stays
  read-only. New registries are written with the current version.
- Entry point for #72: `python -m lib_py.registry_format --yaml <path>` runs
  the migration through the writer and prints
  `registry-format: <from> -> <to>, rows changed: <n>`.

Refs #71

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

.agents/skills/lib_py/agents/sanitize.py

@@ -1,5 +1,6 @@
 import hashlib
 import re
+import shlex
 
 def sanitize_herdr_agent_name(name: str) -> str:
     """
@@ -29,3 +30,14 @@ def sanitize_herdr_agent_name(name: str) -> str:
     
     return s
 
+
+def attach_command(server, name):
+    """The registry row's attach_command (Issue #71).
+
+    A native herdr command, so it works from any shell: the server is always
+    named (`default` too - herdr's env-free default), and the agent by the name
+    herdr registered (sanitize_herdr_agent_name). create, resume and reconcile
+    all write it from here.
+    """
+    return "herdr --session %s agent attach %s" % (
+        shlex.quote(server or "default"), shlex.quote(sanitize_herdr_agent_name(name)))