chore: remove deprecated code and stubs for spec framework pivot (#168)

Remove ~8,300 lines of dead weight in preparation for the adapter
framework pivot:

- Delete deprecated leanspec-mcp Rust crate and @leanspec/mcp npm package
- Delete deprecated leanspec-sync-bridge crate
- Delete orphaned CLI mcp.rs command file (not in mod.rs)
- Delete stub GitHub and ADO providers (all methods returned NotSupported)
- Remove legacy archived-folder migration code and CLI command
- Simplify SpecArchiver and SpecLoader by removing archived/ folder logic
- Update ProviderConfig to only include Markdown variant
- Update doc comments to reflect current state and reference #168

https://claude.ai/code/session_01UoFkEEoLCMScjTLESGdtri

Author: claude