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>

.github/plugin/marketplace.json

@@ -113,12 +113,6 @@
       "description": "Essential prompts, instructions, and chat modes for C# and .NET development including testing, documentation, and best practices.",
       "version": "1.1.0"
     },
-    {
-      "name": "csharp-mcp-development",
-      "source": "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"
-    },
     {
       "name": "database-data-management",
       "source": "database-data-management",