feat(mcp): use a 10-minute sliding TTL for reviewer and merger PR leases (Closes #747)

Reviewer and merger PR leases minted a fixed 120-minute expiry (#407 AC6/AC7)
and derived staleness from two further activity bands: stale at 30 minutes,
reclaimable at 60. A session that died — daemon crash, transport flap, client
restart — therefore kept a PR blocked for an hour before anyone could reclaim
it, and two hours before manual cleanup was sanctioned. #718 records the
resulting deadlock: a merge stalled behind a reviewer lease that had stopped
being live long before it stopped being authoritative.

The flaw was using a long fixed expiry as a proxy for "the owner is probably
still alive" instead of making the owner continuously prove liveness.

Sliding window
--------------
`LEASE_TTL_MINUTES = 10` now governs acquisition, and every write of the lease
marker re-derives `expires_at` from the moment of the write, so each heartbeat
slides the window forward. An actively heartbeating session is never evicted
and has no maximum lifetime; a dead one releases its hold within one TTL.

`LEASE_RENEWAL_MINUTES` is named separately from the acquisition TTL. Renewal
previously had no seam at all: the heartbeat slid the expiry only as a side
effect of re-defaulting the acquisition constant, so the two durations could
not be reasoned about or tuned independently. `format_lease_body` now takes an
explicit `ttl_minutes`, and the heartbeat passes the renewal window rather than
relying on that default.

Merger leases acquire through the same lease-body formatter, so they inherit
the identical window by construction rather than by a parallel constant.

Removal of the reclaim tier
---------------------------
`classify_lease_freshness` no longer returns `reclaimable`. Beyond being an
extra waiting tier, that band is unreachable under a sliding TTL: a heartbeat
stamps `last_activity` and `expires_at` together, so a lease idle for a full
TTL is necessarily already expired. Expiry is now the only takeover gate.

No reclaim path is lost. `find_active_reviewer_lease` already ignores expired
markers, so an expired foreign lease never gated acquisition; and the
`foreign_expired` classification carries the same
`NEXT_ACTION_RELEASE_EXPIRED_LEASE` the retired `foreign_reclaimable` did. The
two updated tests in `test_reviewer_pr_lease.py` assert exactly that: the
classification label changes, the sanctioned next action does not.

`STALE_WARNING_MINUTES` drops to 5 — half the window — so the warning still
fires while the owner can heartbeat and recover.

Diagnostics
-----------
Adds `lease_seconds_remaining`, and the heartbeat tool now returns
`ttl_minutes`, `expires_at`, and `seconds_remaining`, so an operator can
distinguish "held and live" from "held and dying" instead of only seeing that
a lease exists. All additions are additive; no existing key changed.

Also removes `pr_work_lease.DEFAULT_REVIEWER_LEASE_TTL_MINUTES`, a duplicate of
the reviewer TTL with no readers anywhere in the tree, which could only drift.

Legacy markers minted under the old 120-minute TTL still parse and are judged
against their own recorded `expires_at`, so no lease is retroactively expired
by this change.

Full suite: 3425 passed, 6 skipped, 2 failed. Both failures
(`test_issue_702_review_findings_f1_f6`, `test_reconciler_supersession_close`)
reproduce identically on clean master b05075fd25 and are pre-existing.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01QxXHZ7rqXtLgTusngaWgKZ

pr_work_lease.py

@@ -36,7 +36,8 @@ _TERMINAL_CONFLICT_FIX_PHASES = frozenset({"released", "blocked", "done"})
 _ACTIVE_CONFLICT_FIX_PHASES = frozenset({"claimed", "pushing", "pushed"})
 
 DEFAULT_CONFLICT_FIX_TTL_MINUTES = 120
-DEFAULT_REVIEWER_LEASE_TTL_MINUTES = 120
+# The reviewer/merger PR-lease TTL lives in reviewer_pr_lease.LEASE_TTL_MINUTES
+# (#747). A second copy here had no readers and could only drift out of sync.
 
 
 def _parse_timestamp(value: str | None) -> datetime | None: