Package the terminal explorer as cyd

[MicroMilo]

What changed

• Package the terminal explorer as clean-your-data v0.3.0.
• Add the cyd command: cyd opens the current directory and cyd PATH opens a selected path.
• Keep the existing Skill and legacy script paths working through compatibility wrappers.
• Add package smoke coverage, Python 3.9/3.12 CI installation, release metadata, and tag-triggered GitHub distribution builds.
• Reframe the README around the disk browser as the primary product; keep the Agent Skill optional.

Why

The TUI is the product people should be able to download and use without opening the repository. This change makes the explorer installable from GitHub now and ready for PyPI, while preserving the local-only, metadata-first safety boundary.

Validation

• All existing smoke, contract, duplicate, interactive, TUI, cleanup, workspace, and browsing tests pass.
• Package tests pass against python -m clean_your_data and an installed cyd entry point.
• Wheel and sdist build successfully with setuptools 77+.
• Real terminal launch was verified with cyd /tmp and q.
• git diff --check passes.