feat(skills): add dotnet-mcp-builder, deprecate csharp-mcp-server-gen… (#1645)

* feat(skills): add dotnet-mcp-builder, deprecate csharp-mcp-server-generator

Adds a comprehensive skill for building MCP (Model Context Protocol)
servers in C#/.NET against the official ModelContextProtocol 1.x NuGet
packages. Covers both transports (STDIO, Streamable HTTP — SSE is
deprecated) and every primitive in the current MCP spec (2025-11-25):
tools, prompts, resources, elicitation (form + URL mode), sampling,
roots, completions, logging, and MCP Apps. Includes a thin .NET MCP
client reference and testing guidance (MCP Inspector + in-memory
transport for unit tests).

Steers the model toward the current stable 1.x packages instead of the
0.x previews it tends to pin by default, and enforces the STDIO
stdout/stderr trap.

Also deprecates the existing csharp-mcp-server-generator skill, which
predates ModelContextProtocol 1.0 and only covered a subset of the
current spec. Its SKILL.md now redirects users to dotnet-mcp-builder so
existing install URLs keep working without surprises.

* fix: address PR review from aaronpowell

- Delete csharp-mcp-server-generator skill (rather than deprecating it)
- Update mcp-apps.md pitfalls section to reference .NET Tool.Meta type
  instead of the serialized _meta JSON property names
- Rebuild docs/README.skills.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: remove C# MCP development plugin files

* chore: remove csharp-mcp-development plugin entry from marketplace

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugins/csharp-mcp-development/.github/plugin/plugin.json

@@ -1,23 +0,0 @@
-{
-  "name": "csharp-mcp-development",
-  "description": "Complete toolkit for building Model Context Protocol (MCP) servers in C# using the official SDK. Includes instructions for best practices, a prompt for generating servers, and an expert chat mode for guidance.",
-  "version": "1.0.0",
-  "author": {
-    "name": "Awesome Copilot Community"
-  },
-  "repository": "https://github.com/github/awesome-copilot",
-  "license": "MIT",
-  "keywords": [
-    "csharp",
-    "mcp",
-    "model-context-protocol",
-    "dotnet",
-    "server-development"
-  ],
-  "agents": [
-    "./agents/csharp-mcp-expert.md"
-  ],
-  "skills": [
-    "./skills/csharp-mcp-server-generator/"
-  ]
-}

plugins/csharp-mcp-development/README.md

@@ -1,32 +0,0 @@
-# C# MCP Server Development Plugin
-
-Complete toolkit for building Model Context Protocol (MCP) servers in C# using the official SDK. Includes instructions for best practices, a prompt for generating servers, and an expert chat mode for guidance.
-
-## Installation
-
-```bash
-# Using Copilot CLI
-copilot plugin install csharp-mcp-development@awesome-copilot
-```
-
-## What's Included
-
-### Commands (Slash Commands)
-
-| Command | Description |
-|---------|-------------|
-| `/csharp-mcp-development:csharp-mcp-server-generator` | Generate a complete MCP server project in C# with tools, prompts, and proper configuration |
-
-### Agents
-
-| Agent | Description |
-|-------|-------------|
-| `csharp-mcp-expert` | Expert assistant for developing Model Context Protocol (MCP) servers in C# |
-
-## Source
-
-This plugin is part of [Awesome Copilot](https://github.com/github/awesome-copilot), a community-driven collection of GitHub Copilot extensions.
-
-## License
-
-MIT