Feat: Harden governance approval workflow

[rian-be]

Summary

This PR hardens the governance approval workflow so it supports richer operational approval behavior instead of only the initial linear slice.

Added

• role and group targeted approval requirements
• quorum support for grouped approvals
• approval specific expiration handling
• structured approval rejection reasons
• domain specific approval exceptions
• approval workflow example coverage for multi actor and quorum scenarios

Changed

• approval workflow runtime split into validator, decision executor, expiration executor, and guarded persistence
• approval requirements can now carry role, group, quorum, and expiration metadata
• request approval decisions now record quorum satisfaction and expiration explicitly
• approval docs and ADRs updated to reflect the hardened semantics

Result

Approval workflow is now expressive enough for multi actor governance flows while staying request centric and auditable.

Testing

• dotnet build src/ModularityKit.Mutator.Governance.csproj -c Release
• dotnet build Examples/Governance/ApprovalWorkflow/ApprovalWorkflow.csproj -c Release
• dotnet test Tests/ModularityKit.Mutator.Governance.Tests/ModularityKit.Mutator.Governance.Tests.csproj -c Release

Linked Issues

• Closes [Task]: Harden governance approval workflow semantics #17

Checklist

• Approval workflow supports grouped and quorum based approvals
• Approval failures use domain specific exceptions and structured rejection reasons
• Approval expiration is explicit and auditable
• Tests cover multi actor approval behavior
• Example coverage demonstrates non trivial approval scenarios