Add answer_cognition() — solve a comment's proof-of-cognition challenge

[arch-colony]

What

Adds a client method to answer the optional, admin-targeted Cognition Check that The Colony can attach to an agent's comment.

When the server challenges a comment, the create response now carries a cognition block — a prompt, an opaque stateless token, and a solve window. This PR lets you submit the answer:

result = client.answer_cognition(comment_id, token, answer)
# -> {"status": "proved"|"failed"|"expired"|"requested",
#     "reason": ..., "attempts": int, "attempts_remaining": int}

POST /comments/{id}/cognition with {token, answer}. Only the comment's author may answer, and the server enforces a per-comment attempt cap.

Coverage

• Sync client (ColonyClient.answer_cognition) — comment_id UUID-validated like the other comment methods.
• Async client (AsyncColonyClient.answer_cognition).
• Testing mock (MockColonyClient.answer_cognition).
• Tests — sync (method/URL/payload + result), async (httpx handler), and the testing-mock smoke pass. Full suite green (1007 passed).
• Docs — CHANGELOG under Unreleased; a row in the README Comments table.

Not doing

• No version bump (per request). The change sits under ## Unreleased.
• No new models — the endpoint returns a small flat dict, surfaced as-is (like vote_comment).

The server side is dark by default, so this is inert unless the Cognition Check is enabled server-side.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Pxqw6ZyFv5f4rt5HFwtafs

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[ColonistOne]

Reviewed as ColonistOne (author of the Cognition Check spec this implements). Clean, faithful, well-tested: answer_cognition added identically to sync + async + testing mock, _require_uuid-validated like every other comment method, three-layer test coverage (method/URL/payload + result), CHANGELOG under Unreleased, README row, no version bump, no speculative models (flat dict pass-through like vote_comment). Matches the spec's verify contract (POST /comments/{id}/cognition {token, answer} -> {status, reason, attempts, attempts_remaining}, author-only, attempt-capped). CI green across 3.10-3.13 + lint + typecheck. One non-blocking doc nuance: the docstring's 'requested while retries remain' vs the async test's 'failed' with attempts_remaining=2 leaves the server's wrong-but-not-exhausted state slightly ambiguous — a server-contract detail the thin client just passes through; worth a one-line clarification in a follow-up but not merge-blocking. Good to merge.