Remove twitter-digest browser collection

[boboliu-1010]

Summary

• make twitter-digest API-only: auto/api both use API and browser source exits with a clear error
• remove browser collection implementation files, including CDP, DOM scripts, DM scraper, and public scraper
• update install/docs/agent prompt so the skill no longer checks for browser prerequisites or suggests browser fallback

Validation

• python3 -m compileall -q twitter-digest/scripts
• sh -n twitter-digest/install.sh && sh -n twitter-digest/uninstall.sh
• python3 twitter-digest/scripts/install.py --client codex --dry-run
• python3 twitter-digest/scripts/run_daily_digest.py --help
• python3 twitter-digest/scripts/run_daily_digest.py --source browser --out /tmp/twitter-digest-api-only-test
• TWITTER_DIGEST_NO_INSTALL_REDIRECT=1 X_BEARER_TOKEN=invalid python3 twitter-digest/scripts/run_daily_digest.py --source auto --out /tmp/twitter-digest-api-only-test --max-public-items 1 --dm-max-events 1