fix(FN-2581): align done timer duration with task lifecycle timestamps

- Update TaskCard done-duration calculations to use createdAt as the start baseline for completed tasks
- Refactor TaskCard timer tests to assert against TaskStore-backed timestamps and avoid brittle elapsed-time assumptions
- Cover done-state timer behavior with expanded test scenarios for start/end timestamp combinations
- Remove unused provider code in pi-claude-cli to satisfy lint and keep the full test/build gates green

packages/pi-claude-cli/src/provider.ts

@@ -135,7 +135,6 @@ export function streamViaCli(
         newSessionId: !resumeSessionId ? options?.sessionId : undefined,
       });
       const getStderr = captureStderr(proc);
-      const spawnTime = Date.now();
 
       // Register in global process registry for teardown cleanup
       registerProcess(proc);