feat: expand feed coverage and digest modes

[Lyrazhouqy]

Summary

• Run the feed workflow at 23:17 UTC, which is 07:17 Asia/Shanghai, and refresh it when config/default-sources.json changes.
• Increase the X collection cap from 3 to 5 posts per builder.
• Add standard/expanded digest selection, expanded prompts, and digest-expanded.md export.
• Publish the local source additions through config/default-sources.json: 36 X builders, 6 podcasts, and 4 blogs.

Verification

• node --test scripts/generate-feed.test.js scripts/prepare-digest.test.js (17 passed)
• node --check scripts/generate-feed.js scripts/prepare-digest.js scripts/deliver.js
• JSON parsing and git diff --check passed
• Expanded prepare output selected digestMode=expanded and the expanded prompt

Generated feed files are intentionally left for the GitHub Action after merge.