feat(fleet): add CAS-protected stale worker retirement capability

Adds a sanctioned controller/reconciler capability that retires conclusively
stale MCP worker-registry rows through a dry-run-first, compare-and-swap
protected workflow (#980).

The #978 fleet snapshot is read-only, and its registry_revision is seeded with
snapshot_at at second precision, so a CAS gated on it can never pass. That
token is left unchanged; retirement gets its own stable registry_fingerprint
derived exclusively from canonical retirement-relevant registry content, plus a
candidate_fingerprint pinning the approved set. Identical contents observed at
different times produce the same token; row order never affects it; any
retirement-relevant change moves it.

WorkerRegistry.retire_stale_workers performs the whole decision inside one
BEGIN IMMEDIATE transaction: re-read rows, recompute the fingerprint from those
rows, recompute the eligibility plan from those rows (re-reading active workflow
leases), compare the candidate fingerprint, revalidate every target, then retire
each survivor with a guarded UPDATE asserting its status, heartbeat, generation,
session, fencing epoch, and pid are unchanged. Drift retires zero workers and
reports registry_revision_moved or candidate_set_moved; any exception rolls back
and reports transaction_failed, so a partial write is never reported as success.

Retirement requires a full conjunction: active status, complete registry fields,
parsable heartbeat, not live, pid_alive false, expired heartbeat, stale
ownership, canonical repository binding, no identity evidence shared with a live
or unprobeable worker, and no active workflow-lease ownership. Everything else
is preserved with a structured reason code. Retired rows become historical
rather than stale and keep their history; nothing is deleted.

Retirement does not repair untrusted live identity: live workers on legacy
instance identities are preserved and keep their legacy_incomplete_identity
blockers, so the result never claims the fleet became safe.

Exposed as gitea_plan_stale_worker_retirement and
gitea_apply_stale_worker_retirement, restricted to controller/reconciler role
kinds. The Gitea operation gate stays gitea.read because the mutation lands in
the local control-plane registry, matching the #601 lease lifecycle; no new
Gitea write permission is introduced and no author permission is broadened.

Tests: tests/test_issue_980_stale_worker_retirement.py (40 passed, 23 subtests),
including the regression test proving the old snapshot_at derivation moved the
token one second apart while the new registry CAS token does not. Full suite
from the branch worktree matches the master baseline exactly: 28 failed / 6206
passed vs 28 failed / 6166 passed, identical failure set.

Closes #980

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/instance-fleet-identity.md

@@ -189,6 +189,11 @@ mutation. Diagnostic reads remain available where `gitea.read` allows.
 * `mcp_fleet_snapshot` — pure snapshot + classification (#978).
 * `gitea_snapshot_instance_fleet` — sanctioned MCP tool (#978).
 * `gitea_get_runtime_context` — single-process view (not fleet-wide).
+* [stale-worker-retirement.md](stale-worker-retirement.md) — CAS-protected
+  retirement of conclusively stale registrations (#980). Note that the
+  `registry_revision` this snapshot returns is **time-seeded** and unsuitable
+  for compare-and-swap; retirement derives its own stable
+  `registry_fingerprint` from registry content alone.
 
 ## Non-goals