Add SEP-2640 (Skills Extension) protocol support

SEP-2640 (Skills Extension)
is Accepted, but the Python SDK has no support for it. It adds skills/list,
skills/get, and optional resources/directory/read, negotiated under the
io.modelcontextprotocol/skills extension.

We need to add the protocol primitives — a Skills(...) server extension
plus thin client wrappers and SEP-2640 validation — built on the existing
Extension/MethodBinding mechanism (like the shipped Apps extension), with no
schema or dependency changes. Filesystem discovery/caching would stay out of scope
for a higher-level provider.