perf: reuse a deploy-key proof instead of asking the server twice

A key rotation verified the candidate with `git ls-remote`, then immediately ran
preflightCandidate, which threw that result away and ran the same command over a
second SSH connection. Nothing happens between the two calls that could change
the answer, and the proof was already being passed in.

preflightCandidate now uses a proof that established a remote commit and falls
back to verifying when it is handed nothing usable, so it still works as a
standalone gate. Every ssh.exec opens its own connection, so this removes a full
TCP, key exchange and authentication round trip from a rotation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

SOURCE_MANIFEST.txt

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