feat(FN-5143): clear agent logs when task is soft-deleted

Clears agent logs when a task is soft-deleted, with new regression tests covering the end-to-end behavior and documentation updated in the storage guide. The core logic lives in `store.ts` while `store-upsert.test.ts` is updated to reflect the new expectations.

Fusion-Task-Id: FN-5143

packages/core/src/store.ts

@@ -6301,9 +6301,24 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
           },
         });
         this.clearLinkedAgentTaskIds(id, deletedAt);
+        // FN-5143: clear historical agent logs for the soft-deleted task so
+        // downstream readers (evaluator evidence, self-healing diagnostics,
+        // dashboard log views, register-task-workflow-routes) observe zero logs
+        // immediately after deletedAt is set. Atomic with the deletedAt write.
+        this.db.prepare("DELETE FROM agentLogEntries WHERE taskId = ?").run(id);
         this.db.bumpLastModified();
       });
 
+      // FN-5143 defense-in-depth: drop any in-memory buffer entries for this
+      // task. flushAgentLogBuffer() above already ran inside the lock, but a
+      // concurrent appendAgentLog from another async path could re-buffer
+      // before this lock releases; the next flush would still drop them via
+      // ACTIVE_TASKS_WHERE, but filtering here avoids the warn log and keeps
+      // memory bounded.
+      if (this.agentLogBuffer.length > 0) {
+        this.agentLogBuffer = this.agentLogBuffer.filter((entry) => entry.taskId !== id);
+      }
+
       // Remove from cache if watcher is active
       if (this.isWatching) this.taskCache.delete(id);