Add cross-platform release builds (linux/amd64, darwin/arm64)

[kpb]

Goal

• Make it easy to produce release-ready binaries for common platforms.

Scope

• Update the Makefile to add a release build target that cross-compiles:
  • linux/amd64
  • darwin/arm64
• Output binaries to a dist/ directory with clear names, e.g.:
  • wingit-mcp_linux_amd64
  • wingit-mcp_darwin_arm64

Details

• Keep the existing make build target unchanged (local dev build).
• Use standard Go env vars (GOOS, GOARCH).
• Ensure builds are reproducible and do not require CGO.

Acceptance Criteria

• make release-build (or similar) produces both binaries in dist/
• Filenames clearly encode OS and architecture
• No impact to existing build, test, or lint targets

Notes

• Additional platforms (darwin/amd64, windows) can be added later if needed.
• This will support attaching binaries to GitHub Releases for tagged versions.