Bun build pipeline + targets

[johnnyhuy]

Summary

• Add Bun build pipeline and target list.

Context

• Portable executables are a core requirement.

Scope

• Targets: darwin arm64 and x64, windows x64 baseline, linux x64 glibc and musl, linux arm64 glibc and musl.
• Embed version and build metadata for skillet --version.
• Disable runtime .env and bunfig autoload for deterministic builds.

Out of scope

• Code signing and notarization.

Acceptance criteria

• Local build produces all targets with expected naming.
• Build artifacts run and report version correctly.