feat: Add .NET SDK support for Microsoft Agent Framework governance

[imran-siddique]

Problem

The Governance Toolkit is Python-only. Microsoft Agent Framework runs on both Python and .NET, and .NET is the dominant enterprise SDK path. Enterprise teams building on the .NET Agent Framework SDK have no governance layer available.

Impact

• Microsoft's own framework in its primary enterprise language is ungoverned
• Blocks adoption for .NET-first enterprise teams (majority of Microsoft's enterprise base)
• Forces organizations to maintain two stacks: .NET for agents, Python for governance

Proposed Solution

1. Create a agent-governance-dotnet package with C# implementations of core governance primitives
2. Priority modules: policy engine, trust verification, audit logging
3. NuGet package distribution alongside existing PyPI packages
4. Shared policy schema (YAML) works cross-language — only runtime needs porting

Scope

This is a significant effort — likely a separate workstream. Suggesting we start with a minimal viable .NET governance client that can:

• Load and evaluate YAML policies
• Emit audit events to the same audit sink
• Verify Ed25519 agent credentials (using .NET cryptography APIs)

/cc @imran-siddique