Publish mechababs to PyPI

Publish mechababs to PyPI so the bootstrap step can run \`uvx mechababs …\` from a released package instead of a git ref.

## Why
The study-first installer design bootstraps mechababs ephemerally via \`uvx\` (no global install). Today the only source is a git ref (\`uvx --from git+https://github.com/con/mechababs@ mechababs bootstrap\`). A PyPI release gives a cleaner entry point (\`uvx mechababs bootstrap\`), and the installer design should not preclude it.

## Scope
- Package + publish mechababs to PyPI (versioning + release workflow).
- Update the bootstrap docs/UX to prefer \`uvx mechababs\` once released.

## Not in scope
The study-first rewrite itself, which uses a git ref until this lands.

Start by inspecting the package metadata, release configuration, and current bootstrap documentation/UX. Done means mechababs is versioned and published to PyPI, the release workflow supports future releases, and the docs prefer `uvx mechababs bootstrap` while retaining the git-ref approach until release.

Written by the indexing model from the issue text.