CLI: NuGet global tool packaging

Description

Package and publish pptx-mcp as a .NET global tool for distribution via NuGet.

Scope

1. Update PptxMcp.csproj with global tool metadata
2. Add PackAsTool, ToolCommandName, and PackageDescription
3. Create/update README with installation instructions
4. Set up CI/CD for NuGet publish (optional for Phase 1, can be manual)
5. Document dotnet tool install usage

Why This Matters

Distribution: dotnet tool install --global pptx-mcp

Phase 1 MVP focuses on NuGet (reaches .NET developers immediately). Phase 2 can add Scoop/Homebrew support.

Dependencies

Depends on: #98 (dual-mode entry point) and all command issues (#99–#104)

Can start development in parallel with #99–#104; final packaging waits on all commands complete.

Acceptance Criteria

• .csproj marked as tool package
• Publishable to NuGet (local test with dotnet tool install --add-source)
• Installation works: dotnet tool install --global pptx-mcp
• Post-install: pptx-mcp --help works
• README documents installation
• Versioning follows SemVer

Start with PptxMcp.csproj and the existing README, then review the dependency on #98 and commands #99–#104 before packaging. Test a local package with dotnet tool install --add-source, verify pptx-mcp --help, and document the global installation and SemVer expectations in the README.