chore: remove internal files, rewrite README for early dev

- Remove AGENTS.md and skills-lock.json from tracking (internal dev artifacts,
  already in .gitignore). These are not part of the R package and serve only
  as AI-agent scaffolding — not meaningful to end users.

- Rewrite README.md and README_ZH.md following high-quality R package
  conventions with badges, clear install/quick-start sections, pipeline
  overview, and comprehensive function family reference.

- Add early-development warning: breaking changes are extremely likely,
  API is incomplete, bugs are expected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: zorrooz

.gitignore

@@ -43,3 +43,4 @@ a.pdf
 
 # pkgdown local build (deployed from CI to gh-pages branch)
 docs/
+skills-lock.json