Problem: Leader signatures not stored in consensus

[MicBun]

After the leader signs an attestation off-chain, the signature must be stored on-chain in consensus for verifiability.

Acceptance Criteria:

• sign_attestation(request_tx_id, signature) public action implemented
• Action validates caller is current leader (similar to auto_digest pattern)
• Action validates attestation exists
• Action uses @caller to get validator's public key
• Action validates signature field is NULL (not already signed)
• Action updates attestation record with signature, validator_pubkey, signed_height
• All validators execute this action for consensus
• Only callable via execute (write transaction)