Starlark build files, target discovery, and labeled build command

Fabrik’s CLI can now load build files named fabrik.star from each package, scan the workspace for them, expose a glob() helper for relative path expansion, and label every target as //pkg:name. You can list all targets with fabrik targets and build a Rust binary by label, producing the compiled binary under .fabrik/out/<package>/<name> with action-cache reuse.