Add npm/Node.js project detection to clean and stats commands

[Copilot]

Extends project discovery to detect npm projects via package.json and mark their node_modules directories for cleaning/stats reporting, alongside existing .NET/MSBuild project support.

Changes

• DirectoryModels.cs — Add ProjectType.Npm; resolve it from package.json filename in GetProjectType()
• SlnScanner.cs — Add EnumerateNpmFiles() to discover package.json files, filtering out those inside node_modules. Runs alongside existing sln/proj scanning.
• SlnParser.cs — Yield a single ProjCfg (no MSBuild configuration) for npm projects
• CleaningApplication.cs — Short-circuit MSBuild evaluation for npm projects; synthesize a ProjectInfo with OutDir = "node_modules"
• MarkDeleteProcessor.cs — Add NpmDir handler in the (ProjType, DirType) switch, matching the existing VcxDir pattern for direct directory marking

Example output

╭───────┬────────────┬────────────┬───────────────────────────────────╮
│ Files │ Size (KiB) │ Size (MiB) │ Directory                         │
├───────┼────────────┼────────────┼───────────────────────────────────┤
│  4821 │    218,304 │     213.19 │ /repos/my-app/node_modules        │
╰───────┴────────────┴────────────┴───────────────────────────────────╯

Tests

11 new tests in NpmProjectDetectionTests.cs covering detection logic, ProjectType resolution, scanner enumeration, and node_modules-nested package.json exclusion.

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