chore: add auto bump for code formula

[zemaj]

Summary

• bump the Code formula to the 0.4.3 release artifacts so Homebrew users get the latest build
• add an auto-bump-code workflow that watches just-every/code releases & the cron schedule, rewrites Formula/Code.rb, and opens a PR via create-pull-request
• compute new macOS arm64/x86_64 shasums in CI and reference #354 automatically in generated PRs

Testing

• curl -L https://github.com/just-every/code/releases/download/v0.4.3/code-aarch64-apple-darwin.tar.gz | shasum -a 256 # 4d9a1d032ec4d84af442663e70eca3371536352babd0bb31b1168330557772f7
• curl -L https://github.com/just-every/code/releases/download/v0.4.3/code-x86_64-apple-darwin.tar.gz | shasum -a 256 # e6fe3465b96b5bb7bc0108fcb86d564ed360416f2a5ef887eeb29ac6105321bc