Tool refactoring Phase 3: drop Cake.Frosting and Cake.Http packages

[rdeago]

Sub-issue of #228. Depends on Phase 2.

Remove the now-unused Cake package references and verify a clean self-host.

Scope

• Remove <PackageReference Include="Cake.Frosting" /> and <PackageReference Include="Cake.Http" /> from src/Buildvana.Tool/Buildvana.Tool.csproj.
• Remove the corresponding <PackageVersion ... /> entries from Directory.Packages.props.
• Verify a clean dotnet build of the solution: no compile errors, no NU1701 / NU1605 / unresolved-type warnings.
• Verify dotnet bv prepare / restore / build / test / pack self-host locally.
• Verify CI (build-test-pack workflow) succeeds.
• Update any reference to Cake in tool documentation, README snippets, or contributor docs (CHANGELOG entries describing historical behavior may stay).

Acceptance criteria

• Cake.Frosting and Cake.Http no longer appear in Directory.Packages.props or in any csproj.
• dotnet list package --include-transitive for Buildvana.Tool shows no Cake.* packages.
• A repo-wide grep for Cake. in src/**/*.cs returns zero matches.
• dotnet bv build self-hosts both locally and on CI.