Add ssh_key secret parameter to support sync setup_command.

[vorporeal]

Description

The setup_command workflow parameter (added in #16) allows consuming repos to run commands like cargo fetch before sync begins. However, if the source repo has private dependencies that require SSH credentials (e.g. private crates), there's no way to provide them — the reusable workflow runs in a separate job, so the embedding workflow can't set up the SSH agent beforehand.

This adds an optional ssh_key secret to the sync workflow. When provided, it populates the SSH agent (via webfactory/ssh-agent) before the setup command runs, allowing commands like cargo fetch to access private repositories.

Testing

Verified the workflow YAML is syntactically valid. The new step is conditional on secrets.ssh_key != '', so existing consumers that don't pass the secret are unaffected.

Co-Authored-By: Oz oz-agent@warp.dev

[vorporeal]

• Add ssh_key secret parameter to support sync setup_command. #22 👈 (View in Graphite)
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