Feat: Introduce governance package and execution model

[rian-be]

Summary

Introduce ModularityKit.Mutator.Governance as a separate package and define the first governance execution model around mutation requests, pending lifecycle, and request storage.

Added

• Separate ModularityKit.Mutator.Governance package under src/
• Governance abstractions for mutation requests, lifecycle status, pending reasons, request decisions, and request storage
• InMemoryMutationRequestStore as the first governance runtime implementation
• Governance ADR set for package boundaries, mutation requests, pending lifecycle, request storage, and versioned request resolution
• Docs/Roadmap.md and Docs/ExecutionModel.md to document the governance direction and lifecycle model

Changed

• Solution layout now includes the governance package alongside the core runtime
• Core project excludes governance sources to preserve package boundaries
• ADR index is split between core and governance decisions
• Repository documentation now explains the separation between direct execution and governed request lifecycle work
• Governance README now follows the same documentation style as the rest of the repository

Result

The repository now has a clear architectural seam between direct mutation execution and governance-specific request lifecycle work, so future approval, pending execution, versioned resolution, and persistence features can evolve without bloating the core package.

Testing

• dotnet build src/ModularityKit.Mutator.Governance.csproj -c Release
• dotnet build ModularityKit.Mutator.slnx -c Release

Linked Issues

• [Task]: Introduce ModularityKit.Mutator.Governance package #3

Checklist

• PR is focused and does not include unrelated cleanup
• New package boundaries are documented in ADRs
• New behavior follows the existing repository structure under src/
• I followed the Code of Conduct