[MIGRATION]: Update scaffold generator for Rust-native CPEX 0.2 plugins

[lucarlig]

Summary

Update the plugin scaffold generator so new managed plugins can be generated in the Rust-native CPEX 0.2 shape.

Parent epic:

• Convert managed plugins to Rust-native CPEX 0.2 plugins #61

Blocked By

This depends on the CPEX 0.2 Rust plugin contract being stable enough to scaffold against:

• Convert managed plugins to Rust-native CPEX 0.2 plugins #61
• [CHORE]: Upgrade CPEX framework dependency to 0.2 mcp-context-forge#5249
• https://github.com/contextforge-org/cpex/releases

Scope

• Update tools/scaffold_plugin.py for the Rust-native CPEX 0.2 plugin layout.
• Stop generating PyO3 exports for Rust-native plugins.
• Stop generating maturin/pyproject.toml packaging for Rust-native plugins.
• Stop generating Python package modules and Python entry points for Rust-native plugins.
• Generate Cargo metadata suitable for crates.io publishing.
• Generate CPEX 0.2-compatible manifest/config files once the contract is known.
• Generate Rust unit tests and Rust-native CPEX 0.2 integration test scaffolding.
• Update scaffold validation so generated plugins pass repo checks.

Open Questions

• Should the generator support both CPEX 0.1 Python-package and CPEX 0.2 Rust-native modes during transition?
• What final directory layout should Rust-native plugins use?
• What crate naming convention should generated plugins use for crates.io?
• What minimum CPEX 0.2 version should generated plugins target?

Acceptance Criteria

• The scaffold generator can create a Rust-native CPEX 0.2 plugin.
• Generated Rust-native plugins do not include Python modules, PyO3 bindings, maturin config, or Python entry points.
• Generated Cargo metadata is ready for crates.io publishing.
• Generated tests compile and run under the Rust-native plugin path.
• Repo validation covers the Rust-native generated layout.
• Documentation/help text reflects the new scaffold mode and transition behavior.