Verify SSH host keys by default in the Git provider hook

[ephraimbuddy]

The Git hook defaulted strict_host_key_checking to "no" and pointed the known_hosts file at /dev/null, so the SSH transport accepted any host key deploy without verification.

Default to "accept-new" so a host key is trusted on first use and verified on every later connection. A deprecation warning announces the eventual move to "yes" so operators can configure known_hosts ahead of that change.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  claude opus

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[potiuk]

Nice!