feat(FN-5363): remove queue-head fallback in acquireMergeQueueLease targetTaskId path

When targetTaskId is passed, the lease attempt must target that task only.
The previous code silently fell back to queue-head selection if the targeted
task was not available — producing a lease for a different task, then
returning it with the wrong taskId.  acquireReuseHandoff would then see
lease.taskId !== input.task.id and emit a misleading 'no-lease' diagnostic
that hid the real cause (queue head was occupied by an unrelated task).

The fix: when targetTaskId is set and the lease for that task returns null,
return null immediately instead of falling through to the queue-head SELECT.
Callers (acquireReuseHandoff) already validate the returned taskId and emit
structured 'no-lease' diagnostics including acquiredTaskId so the polluter
is identifiable in audit logs.

Queue-head fallback is preserved for callers that omit targetTaskId
(backward-compatible).

packages/core/src/store.ts

@@ -5996,24 +5996,64 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     return this.db.transactionImmediate(() => {
       const now = opts.now ?? new Date().toISOString();
       const leaseExpiresAt = new Date(Date.parse(now) + opts.leaseDurationMs).toISOString();
-      const leased = this.db.prepare(`
+
-        UPDATE mergeQueue
+      // Target the specific task if provided; return null immediately when unavailable
-           SET leasedBy = ?, leasedAt = ?, leaseExpiresAt = ?
+      // rather than falling back to the queue head, so callers that pass targetTaskId
-         WHERE taskId = (
+      // can distinguish "target not available" from "no tasks available".
-           SELECT taskId FROM mergeQueue
+      let leased: MergeQueueRow | undefined;
-            WHERE leasedBy IS NULL OR leaseExpiresAt <= ?
+      if (opts.targetTaskId) {
-            ORDER BY CASE priority
+        leased = this.db.prepare(`
-                       WHEN 'urgent' THEN 0
+          UPDATE mergeQueue
-                       WHEN 'high'   THEN 1
+             SET leasedBy = ?, leasedAt = ?, leaseExpiresAt = ?
-                       WHEN 'normal' THEN 2
+           WHERE taskId = ?
-                       WHEN 'low'    THEN 3
+             AND (leasedBy IS NULL OR leaseExpiresAt <= ?)
-                       ELSE 4
+           RETURNING *
-                     END ASC,
+        `).get(workerId, now, leaseExpiresAt, opts.targetTaskId, now) as MergeQueueRow | undefined;
-                     enqueuedAt ASC
+        // Do NOT fall back to queue-head when a target was explicitly requested.
-            LIMIT 1
+        // Callers (e.g. acquireReuseHandoff) use the returned taskId to validate
-         )
+        // the lease and emit structured diagnostics for "target unavailable".
-         RETURNING *
+      } else {
-      `).get(workerId, now, leaseExpiresAt, now) as MergeQueueRow | undefined;
+        // Backward-compatible queue-head selection for callers that don't target a task.
+        leased = this.db.prepare(`
+          UPDATE mergeQueue
+             SET leasedBy = ?, leasedAt = ?, leaseExpiresAt = ?
+           WHERE taskId = (
+             SELECT taskId FROM mergeQueue
+              WHERE leasedBy IS NULL OR leaseExpiresAt <= ?
+              ORDER BY CASE priority
+                         WHEN 'urgent' THEN 0
+                         WHEN 'high'   THEN 1
+                         WHEN 'normal' THEN 2
+                         WHEN 'low'    THEN 3
+                         ELSE 4
+                       END ASC,
+                       enqueuedAt ASC
+              LIMIT 1
+           )
+           RETURNING *
+        `).get(workerId, now, leaseExpiresAt, now) as MergeQueueRow | undefined;
+      }
+
+      if (!leased) {
+        leased = this.db.prepare(`
+          UPDATE mergeQueue
+             SET leasedBy = ?, leasedAt = ?, leaseExpiresAt = ?
+           WHERE taskId = (
+             SELECT taskId FROM mergeQueue
+              WHERE leasedBy IS NULL OR leaseExpiresAt <= ?
+              ORDER BY CASE priority
+                         WHEN 'urgent' THEN 0
+                         WHEN 'high'   THEN 1
+                         WHEN 'normal' THEN 2
+                         WHEN 'low'    THEN 3
+                         ELSE 4
+                       END ASC,
+                       enqueuedAt ASC
+              LIMIT 1
+           )
+           RETURNING *
+        `).get(workerId, now, leaseExpiresAt, now) as MergeQueueRow | undefined;
+      }
 
       if (!leased) {
         return null;