Feat: add policy composition primitives and examples

[rian-be]

Summary

This PR adds reusable policy composition support and expands examples/tests around composed mutation policies.

Highlights

Policy composition primitives

• Add PolicyCompositionMode for AllOf, AnyOf, and priority-based evaluation.=
• Add policy decision composition and merge behavior.
• Add conflict detection when composed policies modify the same value differently.
• Keep composition metadata visible in the final policy decision.

Composed policy examples

• Add PolicyComposition example with release governance scenarios.
• Demonstrate AllOf approval gates.
• Demonstrate AnyOf emergency override with approval fallback.
• Demonstrate priority based deployment selection.
• Demonstrate explicit owner conflict handling.

Feature flag governance

• Add reusable FeatureFlagGovernancePolicies.CriticalChanges.
• Register one composed governance policy instead of wiring multiple policy classes manually.
• Document composed feature flag governance in the example README.

Test support policies

• Add sample policies for state propagation, approval aggregation, blocking precedence, and composition behavior.

Why

Policy rules often need to be reused as named governance sets instead of being hand wired per scenario.

This adds composition layer so complex policy behavior can be expressed through small focused policies while preserving deterministic ordering, explicit merge behavior, side effects, requirements, metadata, and conflict diagnostics.

Checks

• dotnet build

Closes

• Closes [Task]: Add governance aware policy composition #26