feat: support idempotent clarification dismiss over stdio (A6)

[efecnc]

Depends on altaidevorg/isanagent#104.

Summary

• update the IsanAgent pin for atomic clarification dismissal
• support reply and dismiss through clarification/respond
• reject non-pending or duplicate responses

Test plan

• cargo test -p altai-cli --test serve_stdio clarification_response_rejects_a_non_pending_ticket
• cargo test -p altai-protocol

[amazon-q-developer]

The PR successfully implements idempotent clarification dismissal over stdio. The changes properly extend the clarification/respond endpoint to handle both "reply" and "dismiss" actions, add the required dismiss_clarification method to StdioHost, and include test coverage verifying that duplicate or non-pending dismiss requests are properly rejected. The implementation maintains atomicity through the ClarificationHub's cancel_wait_if_pending method, ensuring idempotent behavior. No blocking issues found.

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