Migrate XML prompt editor into repository

[oywino]

Summary

• migrate the local XML prompt editor app into the repository
• add baseline repository scaffolding for collaboration and maintenance
• add a Codex-specific GitHub CLI wrapper for this workspace

Included

• browser app source files (app.js, app.py, index.html, style.css)
• contributor docs and architecture notes
• pull request and issue templates
• a basic GitHub Actions CI workflow
• .gitignore updates for local Codex GitHub CLI state

Testing

• verified GitHub repository access from Codex with the local gh wrapper
• verified branch push to GitHub
• added CI to compile app.py and validate required static app files on push and PR

Notes

• the workspace-local .codex-gh/ directory is intentionally gitignored because it contains local GitHub CLI auth state