Set up Github Actions CI

[mazi-eth]

Adds a GitHub Actions CI workflow that builds on every push/PR to main using macos-14 + Xcode 15+.

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[lvchenjia]

Nice work getting a CI workflow started. A couple of issues that will break the build:

1. swift test will fail — The Package.swift only declares an executable target with no test target. SwiftPM errors with no tests found; create a target named 'BrowserProxyTests' to add tests and exits non-zero. Either remove this step or add a Tests/ target.

2. Hardcoded Xcode path — sudo xcode-select -s /Applications/Xcode_15.2.app pins to a specific Xcode version. The macos-14 runner ships with Xcode 15.x by default, so this step is unnecessary and will break if the runner image drops Xcode_15.2.app. Consider removing it, or use sudo xcode-select -s /Applications/Xcode.app.

3. Minor: Missing trailing newline at end of file.

Everything else looks good — the trigger events, macos-14 (matches the .macOS(.v14) platform requirement), and swift build -c release (consistent with build.sh) are all correct.