fix: retry failed generate jobs

[reobin]

The generate job treated zero colorschemes as a successful run, permanently advancing the repository watermark.

This change:

• Checks the repository's colors/ tree when generation produces zero results.
• Records an error for repositories that contain colorscheme files or cannot be checked.
• Retries failed generate runs up to three times. A later push resets the retry window.
• Reports genuinely empty repositories separately.

The generated color-data fix should land before this change so retries do not consume their cap against the existing data issue.