feat: add deterministic park-work episode safety skill

[jamestexas]

What changed

Adds the skill-first park-work workflow for safely ending and later resuming human↔agent work episodes.

The implementation provides:

• a pure deterministic receipt/safety fold;
• mechanical check, park, receipt, checkpoint, and resume workflows;
• durable repository-state and PR applicability evidence;
• idempotent receipt retries with immutable completion-contract equality;
• fail-closed behavior for active episodes, conflicts, malformed evidence, and unavailable authoritative history;
• README registration and focused regression tests.

Why

The work board tracks PRs in flight, not agentic work in flight. This gives a provider-neutral episode record without pretending that Rosary currently exposes enough history to prove completion or an atomic active-session claim.

Validation

• 79/79 park-work tests pass;
• py_compile passes;
• scripts/build.sh check passes (50 files);
• git diff --check passes;
• targeted retry-contract and live-Rosary-gate probes pass;
• disposable live Rosary smoke passed receipt write/read-back, conflict rejection, and cleanup;
• independent final review found no scoped findings.

Rosary follow-up beads

• rosary-a6166d: expose command-bound ordered verification history;
• rosary-04faf5: atomic episode claim/lease for active resume;
• rosary-c90d90: canonical bead-store resolution from linked Git worktrees;
• rosary-c91056: accurate Git/jj workspace-checkpoint MCP contract.

The skill intentionally remains fail-closed until those substrate capabilities exist.