feat: add Homebrew tap support

[dylanjha]

Summary

• Adds a homebrew job to the release workflow that automatically generates and pushes an updated Homebrew formula to muxinc/homebrew-tap on each release
• Includes a reference Formula/mux.rb in this repo (not used by the workflow directly)

Setup steps

Before the next release, the following need to be done:

• Create the muxinc/homebrew-tap repository

• Create a GitHub App for cross-repo access

The release workflow needs to push to muxinc/homebrew-tap from this repo. A GitHub App is used to generate a scoped token.

1. Go to GitHub Org Settings > Developer settings > GitHub Apps > New GitHub App
2. Name it something like Mux Homebrew Tap Bot
3. Set the following Repository permissions:
   • Contents: Read & Write (to push formula updates)
4. No webhook URL is needed — uncheck "Active" under Webhooks
5. Under "Where can this GitHub App be installed?", select "Only on this account"
6. Create the app and note the App ID
7. Generate a private key and download it

• Install the GitHub App

1. Go to the app's settings > Install App
2. Install it on the muxinc org
3. Grant access to only the homebrew-tap repository

• Add secrets to muxinc/cli

Add the following repository secrets (Settings > Secrets and variables > Actions):

• HOMEBREW_APP_ID — the App ID from step 2

• HOMEBREW_APP_PRIVATE_KEY — the full contents of the private key .pem file from step 2

• Verify

After merging this PR, tag a release and confirm that:

• The homebrew job runs successfully
• muxinc/homebrew-tap has a Formula/mux.rb with the correct version and SHA256 checksums
• brew tap muxinc/tap && brew install mux works