Sync skill/code packages to S3

[Rader]

Summary:

• Added S3 sync capabilities for skill and agent template code repos to generate and cache zip packages, reducing redundant Git archive generation during downloads.
• Configured downloads to prioritize S3 packages (falling back to Git archive with async S3 backfill) and updated agent skill list to return server-side download URLs.
• Implemented package sync via Git push callbacks/workflows and added cleanup logic to remove S3 packages upon repo deletion.

[Rader]

Depends on #1369

[HaiHui886]

lgtm