Add ContextZIP skill MVP

[junjunjunbong]

Summary

• Add a Codex contextzip skill for preparing local project files for ChatGPT web.
• Add a standard-library Python packer that copies selected files byte-for-byte into a flat .contextzip/upload directory.
• Preserve source-to-output mappings and SHA-256 integrity data in a local manifest outside the upload directory.
• Add deterministic all and current selection modes using paths, Git metadata, .gitignore, size limits, and explicit include/exclude patterns.
• Block common sensitive paths, generated directories, symlinks, and oversized files by default.
• Add installation and usage documentation, unit tests, and GitHub Actions CI for Python 3.10 and 3.12.

Why

ContextZIP is intended to move working context from a local coding environment into ChatGPT web without summarizing, concatenating, or rewriting the original files. This keeps packaging overhead low and lets users reuse an existing ChatGPT subscription without adding an LLM API preprocessing step.

User impact

After installing the repository as a Codex skill, a user can invoke $contextzip. The skill first previews eligible files, then creates a drag-and-drop folder while keeping file contents unchanged. Directory paths are encoded into flat filenames, and extensionless files receive a .txt suffix for web upload compatibility.

Validation

• python3 -m py_compile scripts/pack.py tests/test_pack.py
• python3 -m unittest discover -s tests -v - 8 tests passed
• python3 scripts/pack.py --root . --mode all --max-files 40 --dry-run - selected the expected 6 repository files

Current limitations

• Sensitive-file filtering is path based, not a full content secret scanner.
• File selection is deterministic rather than semantic.
• ContextZIP prepares files locally but does not automate browser upload.