feat: add Homebrew installation support

[gandli]

Summary

Add Homebrew installation support for opencli-rs.

This PR does two things:

• Add Homebrew install instructions to README.md, README.zh.md, and README.ja.md
• Add an update-homebrew job to the release workflow that generates Formula/opencli-rs.rb and pushes it to the Homebrew tap repo on each release tag

The workflow has also been cleaned up in follow-up changes:

• Merge 4 artifact download steps into 1 download-artifact step with merge-multiple: true
• Remove the heredoc indentation workaround and generate the formula directly without the sed hack
• Build release URLs from shell variables only for consistency
• Ignore local-only files like CLAUDE.md and .DS_Store

After setup, users can install via:

brew tap nashsu/opencli-rs
brew install opencli-rs

Repo owner setup

1. Create repo nashsu/homebrew-opencli-rs (public)
2. Add a repository secret named HOMEBREW_TAP_TOKEN in nashsu/opencli-rs
3. Ensure the token can push to nashsu/homebrew-opencli-rs

Validation

• Verified the workflow content and generated formula layout locally
• Verified the generated formula passes Ruby syntax check
• Confirmed the PR is a single squashed commit

Test plan

• Create nashsu/homebrew-opencli-rs
• Add HOMEBREW_TAP_TOKEN
• Push a new release tag and confirm the workflow updates the tap repo
• Verify brew tap nashsu/opencli-rs && brew install opencli-rs works on macOS
• Verify brew install opencli-rs works on Linux