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

gitea_mcp_server.py

@@ -12132,6 +12132,13 @@ def gitea_heartbeat_reviewer_pr_lease(
     verify_preflight_purity(remote, task="review_pr")
     h, o, r = _resolve(remote, host, org, repo)
     auth = _auth(h)
+    # Slide the sliding TTL forward (#747): the heartbeat is the liveness proof,
+    # so renewal is stated explicitly rather than inherited from the acquisition
+    # default.
+    beat_at = datetime.now(timezone.utc)
+    renewed_expiry = beat_at + timedelta(
+        minutes=reviewer_pr_lease.LEASE_RENEWAL_MINUTES
+    )
     body = reviewer_pr_lease.format_lease_body(
         repo=f"{o}/{r}",
         pr_number=pr_number,
@@ -12144,6 +12151,8 @@ def gitea_heartbeat_reviewer_pr_lease(
         candidate_head=candidate_head or session.get("candidate_head"),
         target_branch=session.get("target_branch") or "master",
         target_branch_sha=target_branch_sha or session.get("target_branch_sha"),
+        last_activity=beat_at,
+        ttl_minutes=reviewer_pr_lease.LEASE_RENEWAL_MINUTES,
     )
     comment_url = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
     with _audited(
@@ -12184,6 +12193,13 @@ def gitea_heartbeat_reviewer_pr_lease(
         "phase": phase,
         "comment_id": posted.get("id"),
         "session_lease": updated,
+        # Report the renewed window so an operator can tell "held and live"
+        # from "held and dying" (#747).
+        "ttl_minutes": reviewer_pr_lease.LEASE_RENEWAL_MINUTES,
+        "expires_at": renewed_expiry.replace(microsecond=0)
+        .isoformat()
+        .replace("+00:00", "Z"),
+        "seconds_remaining": reviewer_pr_lease.LEASE_RENEWAL_MINUTES * 60,
         "reasons": [],
     }