Replace GitHub API artifact discovery with a static base URL, verify the
downloaded package by sha256, and record the applied checksum so the
wrapper can report when mling is already up to date.

Add `mling update` to fetch the latest GitHub Actions artifact for the
current platform, stage it as a tarball in the data directory, and apply
it on the next wrapper invocation. The wrapper extracts the staged
package over the installation directory while skipping the running
executable, then removes the staged file.

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.

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.

Add package enable/disable functionality with version matching
for pkg-enable, and fix help text to reflect implemented commands.

Implement `mling uninstall` to remove installed packages by name or
name@version, with shell completion support. Rework install to build
release binaries, copy them plus completion scripts to a per-version
directory under the user data dir.

Move the CLI binary from `src/main.rs` to `src/bin/cli.rs`
and move shared program generation into a new lib.rs module.