Add Homebrew installation support

[vlasky]

Summary

• Add Formula/semtools.rb Homebrew formula that downloads prebuilt binaries per-platform (macOS ARM/Intel, Linux ARM/x86_64)
• Add update-homebrew.yml GitHub Actions workflow that automatically updates formula SHA256 checksums when a new release is published
• Add x86_64-apple-darwin (Intel Mac) target to the release build matrix so Homebrew users on Intel Macs get a native binary
• Update README with brew tap / brew install instructions

How it works

With the formula in this repo, users install via:

brew tap run-llama/semtools https://github.com/run-llama/semtools
brew install semtools

Simpler installation with a tap repo

To enable the shorter:

brew install run-llama/tap/semtools

You would need to create a run-llama/homebrew-tap repo containing just the Formula/ directory. Homebrew auto-resolves run-llama/tap to github.com/run-llama/homebrew-tap. Steps:

1. Create a new repo run-llama/homebrew-tap
2. Copy Formula/semtools.rb into it
3. Update the update-homebrew.yml workflow to push formula updates to that repo instead (requires a PAT or deploy key with write access to the tap repo)

Test plan

• Formula passes brew style (rubocop linting)
• Formula passes brew audit
• Formula installs successfully via brew install on Linux x86_64
• Installed binary runs correctly (semtools --help)
• Formula test passes (brew test)