Add repository scaffold README and gitignore

[xubinrui]

Summary

• add a root .gitignore for common JavaScript and static-web generated files, env files, editor metadata, and OS junk
• expand the placeholder README into a minimal repository stub that documents the current scaffold and contributor expectations

Context

• keeps the change repository-scaffold oriented and intentionally separate from PR Add a polished static OpenAI-compatible chat client #2's application work

Verification

• git diff -- README.md .gitignore
• git status --short
• grep checks for expected README sections and ignore rules