VS 2026: Project scaffold and VSIX configuration

## Description
Create the foundational C# project structure and VSIX configuration for the VS 2026 extension using the new VisualStudio.Extensibility SDK (not legacy VSSDK). This is the base upon which all other VS 2026 features depend.

## Acceptance Criteria
- [ ] C# project created in `vs2026/` or `src-vs2026/` folder
- [ ] Solution file (.sln) configured with NuGet package references
- [ ] VSIX manifest configured with extension metadata
- [ ] VisualStudio.Extensibility SDK packages installed and referenced
- [ ] Extension activation and command registration working
- [ ] Project builds successfully with no errors
- [ ] GitHub Actions CI/CD recognizes VS 2026 project and runs builds

## Implementation Details

### Project Structure
Create folder: `vs2026/` (or `src-vs2026/` if preferred for consistency)

Suggested layout:
```
vs2026/
SquadUI.VS2026.csproj
Properties/
AssemblyInfo.cs
Extension.cs (entry point)
SquadUI.VS2026.sln
```

### VSIX Configuration
- Create extension manifest with metadata:
- Display name: "Squad UI - Visual Studio 2026"
- Description: "Visualize your Squad AI team members and their tasks"
- Version: 0.1.0
- Minimum VS 2026 version requirement
- Configure command table (*.vsct) or command registration via VisualStudio.Extensibility API

### VisualStudio.Extensibility SDK Setup
- Install `Microsoft.VisualStudio.Extensibility` NuGet package (v17.x or latest)
- Do NOT use legacy VSSDK (Microsoft.VisualStudio.SDK)
- Configure MEF component registration for extension services
- Set up extension entry point with proper async activation

### Extension Activation
- Implement `IExtensionService` or equivalent activation interface
- Register with MEF ComponentPartCatalog
- Basic "Hello VS 2026" command to verify activation works
- No tool windows yet (that's Issue #2)

## Size Estimate
**M** (Medium) — 4-6 hours

## Priority
**P1** (Critical Path)

## Dependencies
- None — this is the foundation

## Notes
- Reference Microsoft's [VisualStudio.Extensibility sample projects](https://github.com/Microsoft/VSExtensibility) for project structure
- VS Code extension lives in `src/` as TypeScript; VS 2026 extension is entirely separate C# codebase
- Both extensions share the `.ai-team/` file format and layout, but have independent implementations
- Assigned to: **squad:virgil** (VS 2026 Extension Dev)