chore(at): bump @atproto/api to ^0.19.4, migrate BskyAgent → AtpAgent
- Align @atproto/api across all three package.json files (root, system/, at/)
- Replace deprecated BskyAgent with AtpAgent in 12 .mjs files
- Add at/cli.mjs — unified AT Protocol CLI (ac-at)
- Add PDS contact email (mail@aesthetic.computer) to pds.env.example
- Include AT Protocol integration audit report
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>