feat(ai): add a verified local model artifact store #83

Manage model files as explicit, verifiable application artifacts.

Scope

• Define a versioned model manifest with size, license, runtime compatibility, and checksum.
• Support atomic installation from an explicit download or local-file import.
• Verify the checksum before a model becomes loadable.
• Detect partial or corrupted installs.
• Remove all files for a selected model on request.

Acceptance criteria

• Interrupted installs do not appear ready.
• Duplicate imports do not waste disk space.
• Paths and manifest parsing are covered by tests.
• No model is downloaded automatically.