Rename .NET policy model package

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Rename the existing .NET policy package from Devolutions.NowPolicy to Devolutions.Now.Policy.Model.
• Rename the .NET policy solution, project folders, project files, namespaces, and test project to match the new package identity.
• Update policy README wording to describe the .NET policy model package.
• Update xtask so formatting/build/test automation uses the renamed policy solution and project paths.

Policy NuGet Naming

This PR starts the policy package family under the Devolutions.Now.Policy.* namespace. The intent is for packages under policies/ to cover policy-related code broadly, not only the current package-operation policy document shape.

Proposed policy NuGet roles:

Package	Role
Devolutions.Now.Policy.Model	Admin-authored policy documents and schema-facing model types: policy documents, metadata, rules, constraints, parsing, serialization, and schema helpers. This is the package renamed in this PR.
Devolutions.Now.Policy.Api	Future Agent policy API boundary types: requests, responses, decision payloads, errors, capabilities, and versioning. This keeps API shapes separate from policy document models without introducing a second .Model package.
Devolutions.Now.Policy.Client	Future client implementation that talks to the Agent policy API and depends on Devolutions.Now.Policy.Api.

This keeps the naming centered on policy as the umbrella domain while reserving .Model specifically for admin-authored policy document models.

Tests

• dotnet format .\policies\dotnet\Devolutions.Now.Policy.slnx --verify-no-changes
• dotnet test .\policies\dotnet\Devolutions.Now.Policy.slnx