Add @next formula variants for pre-release testing

## Background

For first-party projects, it's useful to be able to install a pre-release version via Homebrew to get the isolation benefits of a separate install environment without needing to manage a venv manually.

## Approach

Create `Formula/@next.rb` variants (e.g. `Formula/climax-mcp@next.rb`) pinned to the latest pre-release version on PyPI. These install alongside the stable formula and are invoked explicitly:

```bash
brew install get2knowio/tap/climax-mcp@next
```

## Implementation notes

- `@next` formulae are independent of the stable formula — separate class, url, sha256, and resource blocks
- The auto-update workflow from #2 should handle `@next` formulae as a first-party case, triggered when a pre-release is published to PyPI (e.g. version strings matching `*.dev*`, `*rc*`, `*a*`, `*b*`)
- Only create `@next` variants for first-party projects where we control the publish pipeline — third-party pre-releases are not worth tracking

Start by inspecting the existing files under Formula/ and the auto-update workflow described in issue #2. Identify which formulas represent first-party PyPI projects and determine how pre-release versions are detected. Done means eligible projects have independent @next formulae that install alongside stable versions and the update workflow handles their pre-releases.

Written by the indexing model from the issue text.