SEP-2640: Skills Extension #1140

Expected Behavior

The Java SDK should support the SEP-2640 Skills extension, enabling MCP clients to discover and retrieve Agent Skills exposed by MCP servers.

The SDK should provide:

• Schema types and method constants for skills/list, skills/get, and the optional resources/directory/read.
• Client APIs to list skills, retrieve a skill entry by its SKILL.md URI, read skill resources, and—when advertised—read directory resources.
• Stateless server APIs to register skills and expose the required endpoints.
• Support for io.modelcontextprotocol/skills capability negotiation, including the optional directoryRead setting.

Current Behavior

The SDK does not yet support the SEP-2640 Skills extension.

Context

This change adds support for the SEP-2640 Skills extension to the Java SDK, aligning it with the following implementations in other MCP SDKs:

• Python SDK: python-sdk#3485
• C# SDK: csharp-sdk#1856
• Go SDK: go-sdk#1238

I implemented this before realizing that non-trivial changes should be discussed with maintainers first. I would appreciate feedback on the proposed scope and API design, and I am happy to revise the implementation accordingly.