docs: explain Polyester CLJS transition status

[meekmachine]

Summary

• add a detailed CLJS transition architecture note explaining what is currently implemented in CLJS
• summarize the recent merged PRs and what each contributed to the CLJS runtime direction
• document the remaining TypeScript compatibility surfaces and the missing runtime stream/Effect boundary
• add namespace-level inline comments to every CLJS source file explaining ownership, host boundaries, and what should not move into the agency
• add AGENTS.md guidance so future agents update repo instructions when architecture/process gaps show up
• link the architecture note from the README

Validation

• git diff --check
• npm run typecheck
• npm test
• npm run test:cljs

Refs #15