Add snitch why reverse dependency lookup

[Cellcote]

Summary

• New snitch why <package> [project|solution] command that walks each project's project.assets.json and prints every path from a direct reference (PackageReference or ProjectReference) down to the requested package. Output is a per-project Spectre tree with shared prefixes merged; project-ref hops are tagged (project) so they're easy to distinguish from NuGet packages.
• Replaces running dotnet nuget why per project when chasing a vulnerable or unwanted transitive package — one command covers the whole solution.
• Reuses the existing ProjectBuilder so --tfm, --skip, and solution/project discovery all behave the same as snitch analyze.

Example

> snitch why Newtonsoft.Json src/Snitch.Tests.Fixtures/Baz/Baz.csproj

Baz (netstandard2.0)
└── Bar 1.0.0 (project)
    └── Foo 1.0.0 (project)
        └── Newtonsoft.Json 12.0.1

Found 1 path(s) to Newtonsoft.Json across 1 project(s).

Test plan

• dotnet test src/Snitch.Tests/Snitch.Tests.csproj — all 18 tests pass (6 new + 12 existing)
• snitch why Newtonsoft.Json Foo.csproj shows direct ref
• snitch why Newtonsoft.Json Baz.csproj walks Baz → Bar → Foo → Newtonsoft.Json via project refs
• snitch why Microsoft.NETCore.Platforms Foo.csproj walks NETStandard.Library → Microsoft.NETCore.Platforms
• snitch why System.Text.Json Foo.csproj reports "No dependency paths"
• snitch why newtonsoft.json … matches case-insensitively
• snitch why Newtonsoft.Json Snitch.Tests.Fixtures.sln finds paths in Foo, Zap, Thud, Thuuud

🤖 Generated with Claude Code