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>

docs/README.skills.md

@@ -121,7 +121,6 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md#adding-skills) for guidelines on how to
 | [creating-oracle-to-postgres-migration-integration-tests](../skills/creating-oracle-to-postgres-migration-integration-tests/SKILL.md)<br />`gh skills install github/awesome-copilot creating-oracle-to-postgres-migration-integration-tests` | Creates integration test cases for .NET data access artifacts during Oracle-to-PostgreSQL database migrations. Generates DB-agnostic xUnit tests with deterministic seed data that validate behavior consistency across both database systems. Use when creating integration tests for a migrated project, generating test coverage for data access layers, or writing Oracle-to-PostgreSQL migration validation tests. | None |
 | [csharp-async](../skills/csharp-async/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-async` | Get best practices for C# async programming | None |
 | [csharp-docs](../skills/csharp-docs/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-docs` | Ensure that C# types are documented with XML comments and follow best practices for documentation. | None |
-| [csharp-mcp-server-generator](../skills/csharp-mcp-server-generator/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-mcp-server-generator` | Generate a complete MCP server project in C# with tools, prompts, and proper configuration | None |
 | [csharp-mstest](../skills/csharp-mstest/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-mstest` | Get best practices for MSTest 3.x/4.x unit testing, including modern assertion APIs and data-driven tests | None |
 | [csharp-nunit](../skills/csharp-nunit/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-nunit` | Get best practices for NUnit unit testing, including data-driven tests | None |
 | [csharp-tunit](../skills/csharp-tunit/SKILL.md)<br />`gh skills install github/awesome-copilot csharp-tunit` | Get best practices for TUnit unit testing, including data-driven tests | None |
@@ -141,6 +140,7 @@ See [CONTRIBUTING.md](../CONTRIBUTING.md#adding-skills) for guidelines on how to
 | [documentation-writer](../skills/documentation-writer/SKILL.md)<br />`gh skills install github/awesome-copilot documentation-writer` | Diátaxis Documentation Expert. An expert technical writer specializing in creating high-quality software documentation, guided by the principles and structure of the Diátaxis technical documentation authoring framework. | None |
 | [dotnet-best-practices](../skills/dotnet-best-practices/SKILL.md)<br />`gh skills install github/awesome-copilot dotnet-best-practices` | Ensure .NET/C# code meets best practices for the solution/project. | None |
 | [dotnet-design-pattern-review](../skills/dotnet-design-pattern-review/SKILL.md)<br />`gh skills install github/awesome-copilot dotnet-design-pattern-review` | Review the C#/.NET code for design pattern implementation and suggest improvements. | None |
+| [dotnet-mcp-builder](../skills/dotnet-mcp-builder/SKILL.md)<br />`gh skills install github/awesome-copilot dotnet-mcp-builder` | Build Model Context Protocol (MCP) servers in C#/.NET against the current ModelContextProtocol 1.x NuGet packages. Especially helps with cases the model often gets wrong without guidance — stale preview versions (it tends to pick 0.3 or 0.4 preview), MCP Apps (interactive UI rendered in the host), elicitation URL mode, per-session HTTP wiring, OAuth and reverse-proxy deploy specifics, and debugging concrete MapMcp / STDIO / Streamable-HTTP errors. Also covers the routine work — STDIO and Streamable HTTP transports (SSE is deprecated), tools, prompts, resources, sampling, roots, completions, logging — and a basic .NET MCP client. Trigger when the user says or implies any .NET MCP server work: ModelContextProtocol, McpServerTool, MapMcp, WithStdioServerTransport, "MCP server in C#", "MCP tool in dotnet", "expose this as MCP", or names a primitive (prompt/resource/elicitation/MCP App) in a .NET context. Skip for MCP work in other languages. | `references/client.md`<br />`references/elicitation.md`<br />`references/mcp-apps.md`<br />`references/packages.md`<br />`references/prompt-primitive.md`<br />`references/resource-primitive.md`<br />`references/roots.md`<br />`references/sampling.md`<br />`references/server-features.md`<br />`references/testing.md`<br />`references/tool-primitive.md`<br />`references/transport-http.md`<br />`references/transport-stdio.md` |
 | [dotnet-timezone](../skills/dotnet-timezone/SKILL.md)<br />`gh skills install github/awesome-copilot dotnet-timezone` | .NET timezone handling guidance for C# applications. Use when working with TimeZoneInfo, DateTimeOffset, NodaTime, UTC conversion, daylight saving time, scheduling across timezones, cross-platform Windows/IANA timezone IDs, or when a .NET user needs the timezone for a city, address, region, or country and copy-paste-ready C# code. | `references/code-patterns.md`<br />`references/timezone-index.md` |
 | [dotnet-upgrade](../skills/dotnet-upgrade/SKILL.md)<br />`gh skills install github/awesome-copilot dotnet-upgrade` | Ready-to-use prompts for comprehensive .NET framework upgrade analysis and execution | None |
 | [doublecheck](../skills/doublecheck/SKILL.md)<br />`gh skills install github/awesome-copilot doublecheck` | Three-layer verification pipeline for AI output. Extracts verifiable claims, finds supporting or contradicting sources via web search, runs adversarial review for hallucination patterns, and produces a structured verification report with source links for human review. | `assets/verification-report-template.md` |