feat: add v2 task control and asynchronous isolated workers

include/os_compat.h

@@ -130,6 +130,9 @@ void os_set_cancel_flag(volatile int *flag);
 volatile int *os_get_cancel_flag(void);
 void os_set_run_timeout(int seconds); /* this thread; 0 = default */
 int os_get_run_timeout(void);
+typedef struct { void (*write)(const char *, size_t, void *); void *userdata; } os_output_observer_t;
+void os_set_output_observer(os_output_observer_t *observer);
+void os_observe_output(const char *data, size_t n);
 
 /* Long-lived child with pipes on stdin/stdout (MCP servers). stderr goes
  * to stderr_path (NULL = discarded). env_add: "KEY=VALUE" entries added to