feat(agent): automate FluentKit skill contracts

[NoobNotFound]

Summary

• Replace the release-specific FluentKit API skill with a permanent bootstrap skill.
• Embed the exact API model, documentation, references, manifest, and agent instructions in every FluentKit.Blazor package.
• Expose package-local contract paths through stable buildTransitive MSBuild properties.
• Generate and verify the contract automatically during dotnet pack.
• Add Unix/PowerShell resolvers and one-time checksum-validated installers.
• Convert API smoke testing to local-package consumption and add cross-platform contract failure tests.
• Update PR and release workflows to verify, smoke-test, and publish the bootstrap ZIP/checksum automatically.
• Bump the first contract-bearing package version to 0.2.6.

Validation

• API generation and freshness verification
• NuGet pack and package-contract verification against the built DLL
• Unix resolver and local-package smoke test
• Contract failure scenarios, including missing/incomplete/tampered contracts, paths with spaces, and locked restore failure
• 20 FluentKit unit tests
• Shell, PowerShell, YAML, and skill validation

The full solution build is environment-blocked locally by an unavailable iOS simulator runtime; the FluentKit library and test suite pass.