[dotnet-ai] Plugin scaffold

## Parent Issue

Part of the `dotnet-ai` plugin proposal: #225

## Summary

Add the `dotnet-ai` plugin scaffold to the repository. This creates the directory structure, plugin manifest, CODEOWNERS entry, and marketplace.json registration — establishing the plugin so that subsequent skill PRs have a home.

## Deliverables

```
plugins/dotnet-ai/
plugin.json
agents/.gitkeep
skills/.gitkeep
tests/dotnet-ai/.gitkeep
```

Updates to existing files:
- `.claude-plugin/marketplace.json` — add `dotnet-ai` plugin entry
- `.github/CODEOWNERS` — add ownership entries for `plugins/dotnet-ai/` and `tests/dotnet-ai/`

## plugin.json

```json
{
"name": "dotnet-ai",
"version": "0.1.0",
"description": "AI and ML skills for .NET: technology selection, LLM integration, ML.NET, vector search, RAG, ONNX inference, and agentic workflows.",
"skills": "./skills/",
"agents": "./agents/"
}
```

## marketplace.json Addition

```json
{
"name": "dotnet-ai",
"source": "./plugins/dotnet-ai",
"description": "AI and ML skills for .NET: technology selection, LLM integration, ML.NET, vector search, RAG, ONNX inference, and agentic workflows."
}
```

## Dependencies

None — this is the first PR and unblocks all subsequent skill PRs.

## Acceptance Criteria

- [ ] `plugins/dotnet-ai/plugin.json` exists and is valid JSON
- [ ] `plugins/dotnet-ai/agents/` and `plugins/dotnet-ai/skills/` directories exist
- [ ] `tests/dotnet-ai/` directory exists
- [ ] `.claude-plugin/marketplace.json` lists the `dotnet-ai` plugin
- [ ] `.github/CODEOWNERS` has entries for the new paths

Start by comparing an existing plugin directory with .claude-plugin/marketplace.json and .github/CODEOWNERS to follow repository conventions. Add the listed dotnet-ai and tests/dotnet-ai files and directories, then validate plugin.json and confirm the marketplace and ownership entries satisfy every acceptance criterion.

Written by the indexing model from the issue text.