Add the `class-add` command to create project classes from
templates registered in `.mling/classes.toml`. Supports name
derivation (snake_case, PascalCase, etc.), template expansion,
and shell completions.

Allow `proj-init` to accept a `<ref>@<variant>` template source
that resolves and caches templates from a git repository, falling
back to local directories for plain paths.

Implement `mling proj-init` to generate projects from zip templates with
checklist-based configuration. Extracts template archive, moves
checklist for user editing, and expands `tmpl_`-prefixed files using
`just_template` with boolean rule evaluation from `rule.toml` for file
hides and display toggles.