fix(triage): prevent orphaned deps when splitting tasks + detect worktree drift

Root cause: during a triage split the AI could set a child task's
`dependencies` to the parent id. The parent is hard-deleted after the split,
and the scheduler's dep check treats a missing id as unmet — permanently
blocking the dependent. This stranded FN-2164 behind the deleted FN-2163.

- core/store.deleteTask: refuse to delete when any live task still has the id
  in its `dependencies` array. Throws TaskHasDependentsError listing dependents
  so callers can rewrite or recover. Covers the triage-split path and any
  future caller.
- engine/triage task_create: validate each proposed dependency before creating
  a child — reject the parent id, reject unknown task ids, allow siblings
  created earlier in the same split or pre-existing tasks.
- engine/triage split cleanup: wrap the parent deleteTask in try/catch that
  keeps the parent alive (safer than stranding dependents) and logs the reason.
- engine/triage prompts: both the mandatory-split and proactive-split prompts
  now explicitly state that subtask deps must never reference the parent.
- dashboard/routes /subtasks/create-tasks: reject parent-id deps, drop unknown
  deps with an audit log entry, surface parentTaskCloseError + droppedDependencies
  in the response instead of silently swallowing them.
- engine/executor: on execute entry, detect the drift state (in-progress task
  with no worktree) and emit a loud log + task log entry; the existing
  fresh-worktree path then recovers. Prevents silent "operating without a
  worktree" behavior that we saw on FN-2152.

Tests:
  core:      2907/2907 pass (+5 new, incl. deleteTask guard regression)
  engine:    2554/2554 pass (+17 new, incl. task_create dep validation)
  dashboard: 9064/9064 pass (+2 new for /subtasks/create-tasks).

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

packages/core/src/db.ts

@@ -146,6 +146,7 @@ CREATE TABLE IF NOT EXISTS tasks (
   mergeRetries INTEGER,
   workflowStepRetries INTEGER,
   recoveryRetryCount INTEGER,
+  taskDoneRetryCount INTEGER DEFAULT 0,
   nextRecoveryAt TEXT,
   error TEXT,
   summary TEXT,
@@ -1623,6 +1624,15 @@ export class Database {
       });
     }
 
+    if (version < 41) {
+      // Tracks self-healing auto-requeues of tasks that failed because the agent
+      // exited without calling task_done with partial step progress. Bounded so
+      // a persistently-broken task cannot loop forever.
+      this.applyMigration(41, () => {
+        this.addColumnIfMissing("tasks", "taskDoneRetryCount", "INTEGER DEFAULT 0");
+      });
+    }
+
   }
 
   /**