Replace cargo-dist with simplified release workflow (#263)

Might be easier to look at the new GHA by itself:
https://github.com/Gusto/scope/blob/867b809cdae95c4025d59e405316534f730ba1f1/.github/workflows/release.yml

The last release, for reference:
https://github.com/Gusto/scope/releases/tag/v2026.1.12

## Summary

Replaces cargo-dist and cargo-release with a streamlined release
workflow:

- Each push to `main` merges into a `release` branch, bumps the version
to `YYYY.MM.DD.HHmm`, tags, and builds
- Removes ~450 lines of cargo-dist generated workflow code
- Drops shell installer, Linux packages (.deb/.rpm), and DotSlash
generation—just tarballs now
- Uses `cross` for Linux targets, native cargo for macOS ARM64

## Design Decisions

- **Single merge commit for releases**: Version bump happens in the
merge commit itself, not a separate commit
- **`0.0.0-dev` on main**: Main branch always shows development version;
release branch carries real versions
- **No GitHub App token**: Default `GITHUB_TOKEN` suffices since we only
push to `release` branch, not `main`
- **Four targets**: aarch64-apple-darwin, x86_64-linux-gnu,
x86_64-linux-musl, aarch64-linux-gnu (dropped x86_64-apple-darwin given
declining Intel Mac usage)

## Setup Required

After merging, create the release branch once:

```bash
git checkout main && git checkout -b release && git push -u origin release
```

Assisted-by: Claude

Author: kejadlen