Fold GatesFailed auto-retry into the block subscriber's shared budget

Code review of 1185 (merge 0f4b0c95) found the retry subscriber's central
invariant did not hold: the MergeFailure->Merge bounce caused by its own
retry reset both its attempt counter and the block subscriber's counter,
so the shared merge_failure_block_threshold budget was unreachable and a
deterministic gates failure retried forever.

One subscriber now owns one counter driving both policies:

- Counter survives PipelineEvent::MergeRetryStarted bounces (finding 1);
  a third consecutive failure blocks even with retries in between.
- Mixed failure kinds share the single budget (finding 5).
- Retries respect recovery: no counting or scheduling while a mergemaster
  is active, and perform_auto_retry re-checks before firing (finding 2).
- perform_auto_retry applies the same eligibility gates as
  assign_merge_stage (review hold, frozen, blocked, unmet deps) so freeze
  now stops a retry loop (finding 4).
- Per-story scheduling generations invalidate stale sleeping timers
  (finding 6).
- One-shot startup scan schedules a catch-up retry for stories already
  parked in GatesFailed, so restarts no longer strand them (finding 3);
  kept out of the periodic reconciler to avoid re-retrying exhausted
  stories every tick.
- Chat is notified only after the merge actually starts; a failed trigger
  logs instead of claiming a retry ran (finding 7).
- Config reads moved onto spawn_blocking (finding 8, bug 1170 class).

Deletes merge_failure_retry_subscriber.rs; notification plumbing
(WatcherEvent::MergeAutoRetry et al) is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fHdm92yjvguPi2LiXfLB9

server/src/agents/pool/auto_assign/mod.rs

@@ -4,10 +4,9 @@
 mod auto_assign;
 mod backlog;
 mod merge;
-/// TransitionFired subscriber that auto-blocks stories after N consecutive MergeFailure transitions.
+/// TransitionFired subscriber owning the consecutive-MergeFailure budget:
+/// auto-retries GatesFailed below the threshold, auto-blocks at it.
 pub(crate) mod merge_failure_block_subscriber;
-/// TransitionFired subscriber that auto-retries GatesFailed merge failures after a delay.
-pub(crate) mod merge_failure_retry_subscriber;
 /// TransitionFired subscriber that auto-spawns mergemaster on ConflictDetected merge failures.
 pub(crate) mod merge_failure_subscriber;
 mod pipeline;
@@ -24,10 +23,6 @@ pub(crate) use merge_failure_block_subscriber::reconcile_merge_failure_block;
 /// Re-export for `startup::tick_loop`.
 pub(crate) use merge_failure_block_subscriber::spawn_merge_failure_block_subscriber;
 /// Re-export for `startup::tick_loop`.
-pub(crate) use merge_failure_retry_subscriber::reconcile_merge_failure_retry;
-/// Re-export for `startup::tick_loop`.
-pub(crate) use merge_failure_retry_subscriber::spawn_merge_failure_retry_subscriber;
-/// Re-export for `startup::tick_loop`.
 pub(crate) use merge_failure_subscriber::reconcile_merge_failure;
 /// Re-export for `startup::tick_loop`.
 pub(crate) use merge_failure_subscriber::spawn_merge_failure_subscriber;