fix: use release app for Sourcebot sync

[brendan-kellam]

Summary

• use the existing release GitHub App credentials for the Sourcebot sync dispatch
• retain the existing token scope and repository restriction

Testing

• git diff --check

---

Note

Low Risk
Credential swap only; dispatch target and permissions are unchanged.

Overview
The Sync Sourcebot Zoekt Dependency workflow now authenticates with the release GitHub App (RELEASE_APP_ID / RELEASE_APP_PRIVATE_KEY) instead of dedicated Sourcebot sync app secrets.

Token scope is unchanged: same sourcebot-dev owner, sourcebot repo, and contents: write permission for the repository dispatch step.

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