fix(ci): pin webfactory/ssh-agent action to immutable commit SHA

[SuhJae]

Motivation

• Mitigate a supply‑chain risk where the deploy workflow referenced webfactory/ssh-agent@v0.9.0 (a mutable tag) while loading secrets.SSH_PRIVATE_KEY, which could allow secret exfiltration if the tag is retargeted or upstream is compromised.

Description

• Replace the mutable tag in .github/workflows/deploy.yml with the action's immutable commit SHA webfactory/ssh-agent@dc588b651fe13675774614f8e6a936a468676387 while preserving the same behavior (corresponds to v0.9.0).

Testing

• Verified the workflow file was updated using automated file checks and confirmed the uses: reference now contains the immutable SHA; no unit or integration tests were executed.

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Codex Task