v1.21.7 — agent-sync

• acquire refuses the shared fallback identity where hooks know their session. From
  a shell with no CLAUDE_SESSION_ID, acquire won the lease under the shared fallback,
  while the PreToolUse guard — which runs with the session id — refused every edit with
  "this run holds no lease" (reported 2026-10-11 from a linked worktree). When any session
  stamp is in reach, acquire now exits 2 and names the remedy (CLAUDE_SESSION_ID, or
  AGENT_SYNC_RUN_ID for a non-Claude caller). Where no hook stamps sessions, nothing
  changes: CLI and hook share the fallback and agree.
• A linked worktree reads the main checkout's session stamp. A session started in the
  main checkout stamped that checkout's state dir; the walk from a worktree read only its
  own and fell back to the shared identity. The walk now reads both.

Two cases in test/hooks_session_test.py, both watched failing first.

• A direct guard <path> in a repository with no coordination allows. guard.sh
  already exited early there, but agent_sync.py guard <path> called directly still
  exited 2 with a lease message no lease could satisfy. Recovered from the unmerged
  2026-09-11 branch: a missing config is NotCoordinated, an answer the guard allows;
  an unreadable config still denies. test/guard_uncoordinated_test.py (4 cases, 2
  failing on 1.21.6) joins npm test.