fix(pi): converge worker failures and thinking state

tests/unit/pi-subagent.test.ts

@@ -160,7 +160,7 @@ describe('Pi subagent scheduler', () => {
 
   it('aborts every unfinished child with the parent and leaves no permit or process lease', async () => {
     const processBudget = new PiProcessBudget(8);
-    const stopped: string[] = [];
+    const stopped: Array<{ taskId: string; reason: string }> = [];
     const scheduler = new PiSubagentScheduler({
       processBudget,
       openChild: async (input) => ({
@@ -173,7 +173,7 @@ describe('Pi subagent scheduler', () => {
           });
           return { summary: 'unreachable' };
         },
-        async stop() { stopped.push(input.taskId); },
+        async stop(reason) { stopped.push({ taskId: input.taskId, reason }); },
       }),
     });
     const controller = new AbortController();
@@ -186,6 +186,7 @@ describe('Pi subagent scheduler', () => {
     const result = await flight;
     expect(result.details.tasks.map(({ status }) => status)).toEqual(['aborted', 'aborted']);
     expect(stopped).toHaveLength(2);
+    expect(stopped.map(({ reason }) => reason)).toEqual(['subagent_abort', 'subagent_abort']);
     expect(processBudget.activeCount).toBe(0);
     await scheduler.close();
   });