Add SEP-2640 conformance tests #152

Is your feature request related to a problem? Please describe.

SEP-2640 implementations need a consistent way to validate that Skills extension behaviour matches the accepted specification.

Describe the solution you'd like

Add conformance coverage for the main SEP-2640 flows, including:

• skills/list
• skills/get
• skill manifest validation
• optional resources/directory/read

Implementation PR:

modelcontextprotocol/conformance#330

Additional context

This work has already been done and merged by @panyam . It is being tracked here for Kanban completeness.