Add `[[hide-dir]]` rule to remove entire directories when their
condition evaluates to true, extending the existing file-level
hide functionality to directory trees.

Support `[[user.toggle-mutex]]` groups in rule.toml to enforce
mutually exclusive toggle options, and derive `program_crate_name`
from `program_name` using snake_case conversion.

Remove `tmpl_` prefix stripping and special `.mling` handling, rendering
all files as templates except metadata files.

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.

Replace the ZIP-based template archive workflow with direct
directory copying. Delete the `package-mling-tmpls` tool and
remove the `zip` dependency.

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.