Lay the groundwork for a GTK/Adwaita frontend by extracting
shared infrastructure into a LumaCore Swift package. Agent
sources and LumaBundleCompiler move to top-level locations
so both the Xcode project and SPM can reference them.
New structure:
- Package.swift: root manifest with LumaCore lib + compiler
- Sources/LumaCore/: engine, persistence, models, and
platform-abstracted token store
- Sources/LumaBundleCompiler/: moved from LumaBundleCompiler/
- Agent/: moved from Luma/Agent/
- LumaGtk/: skeleton GTK app package
