fix: clarify SSH host key verification failures

[whyujjwal]

Summary

• detect deployer/OpenSSH host key verification failures in action output
• emit actionable guidance that names the remote host and explains this is the server host key, not the private key secret
• add focused tests for host parsing and guidance formatting

Validation

• npm test
• npm run typecheck
• npm run build
• npm run format:check

Closes #61