SEP-2640: Skills Extension

Requesting support for SEP-2640 (Skills Extension, Extensions Track, Final), which serves Agent Skills over the existing Resources primitive.

Nothing in the SDK covers it today: no skills/* constants in lib/mcp/methods.rb, no helper module, and no match for skill anywhere under lib/ on main (1.6.0). ROADMAP.md names DPoP, workload identity federation, and the tasks extension as the unimplemented extensions — skills isn't listed.

What a server declaring the extension owes

• skills/list and skills/get — both MUST
• resources/directory/read — optional, gated behind directoryRead: true
• the SEP-2133 capabilities.extensions fragment io.modelcontextprotocol/skills, alongside a declared resources capability
• on 2026-07-28, skills/list results carry the SEP-2549 ttlMs/cacheScope hints

Prior art in the other official SDKs

• python-sdk: modelcontextprotocol/python-sdk#3486, modelcontextprotocol/python-sdk#3485
• typescript-sdk: modelcontextprotocol/typescript-sdk#2798, modelcontextprotocol/typescript-sdk#2818
• go-sdk: modelcontextprotocol/go-sdk#1238, modelcontextprotocol/go-sdk#1240
• csharp-sdk: modelcontextprotocol/csharp-sdk#1863, modelcontextprotocol/csharp-sdk#1864

Notes

MCP::Apps looks like the shape this would take: a helper module for an Extensions-Track SEP, with Capabilities#support_extensions for negotiation and the resources primitive underneath.

Most of it can be assembled today from define_custom_method plus resources_read_handler, and a custom method's result does get resultType stamped on the modern path. The SEP-2549 hints don't follow: Server::CACHEABLE_RESULT_METHODS is frozen and consulted internally, so a hand-rolled skills/list has to merge ttlMs/cacheScope into its own result. Worth folding into whatever first-class support lands.