v0.6.0 test release

gate() includes verbose instructions for agents

When gate() returns challenge_required, the response now includes an
'instructions' field explaining:
- This is a verification challenge
- How to read the prompt and determine the answer
- Exact JSON fields to include (challenge_token + answer)
- That other request fields can be sent alongside (open-ended)
- Whether a persistent token will be issued (varies by mode)
- How to use the token on future requests (Authorization header)

Two variants:
- persistent=True: instructions mention saving and reusing the token
- persistent=False: instructions note every request needs a challenge

78/78 tests passing (3 new instruction tests)