BREAKING CHANGE: `#[completion]` functions must now take `ShellContext`
by value instead of `&ShellContext`. Reference parameters are reserved
for resource injection.

Remove the `pack!`, `pack_err!`, `pack_structural!`, and
`pack_err_structural!` macros, replacing all pipeline type definitions
with `#[derive(Grouped)]` and `#[derive(Grouped, Wrap)]` attributes.
This changes the generated struct shape from named-field structs with an
`inner` field to tuple structs accessed via `.0`, and removes the
auto-generated `name` and `info` fields from error types.

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.