Refactor `find_project_root` to use `deverbatim` on canonicalized paths,
removing the `\\?\` prefix on Windows to ensure proper path handling.

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.

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

Add `!=` operator support in rule expressions and ensure the entire
expression is consumed during parsing, invalidating any trailing
garbage tokens that would previously be silently ignored.

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.