fix(engine): prevent auto-merge cooldown loop on unresolvable conflicts

Tasks were getting stuck in `in-review` forever when auto-merge could not
resolve conflicts within MAX_AUTO_MERGE_RETRIES. The conflict-exhaustion
branch silently cleared `status` (no error, no log entry, no comment),
and the 30-min cooldown sweep would reset retries and re-attempt the
same impossible merge — looping silently with no user-facing surface.

Why:
- FN-2918 and FN-2903 both spent hours in this loop with no error/comment
  visible on the task. The only log evidence was repeated
  "Auto-merge retry cooldown elapsed (30m idle)" entries with no
  follow-up outcome.

How to apply:
- Every merge failure now writes a `<Manual|Auto>-merge failed: <msg>`
  entry to the task log so the dashboard surfaces the reason.
- Conflict-retry exhaustion now bounces the task back to `in-progress`
  with a comment + log entry so the executor re-rebases against main
  and retries — mirroring the verification-failure-bounce pattern.
- New `mergeConflictBounceCount` task field caps outer bounces
  (`MAX_MERGE_CONFLICT_BOUNCES = 2`); past the cap, the task is parked
  in `in-review` with `status="failed"` and a follow-up triage task is
  created so a human can resolve the conflict manually.
- Non-conflict and non-direct-strategy errors now also set
  `status="failed"` so the cooldown sweep can't re-pick them up.
- `canMergeTask` skips tasks with `status="failed"` so terminal
  failures (verification cap, bounce cap, non-conflict error) are no
  longer eligible for cooldown re-attempts.

Schema migration v52 adds the `mergeConflictBounceCount` column.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/db.ts

@@ -86,7 +86,7 @@ export function probeFts5(db: DatabaseSync): boolean {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 51;
+const SCHEMA_VERSION = 52;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -1949,6 +1949,16 @@ export class Database {
       });
     }
 
+    // Outer auto-merge bounce counter so the cooldown sweep can't loop forever
+    // on a task whose conflicts can't be auto-resolved. Capped by
+    // MAX_MERGE_CONFLICT_BOUNCES in project-engine.ts; once reached, the task
+    // is parked in in-review with status="failed" and a follow-up is created.
+    if (version < 52) {
+      this.applyMigration(52, () => {
+        this.addColumnIfMissing("tasks", "mergeConflictBounceCount", "INTEGER DEFAULT 0");
+      });
+    }
+
   }
 
   /**