Align Skills extension with updated spec (2026-07-28): directory caching and skill metadata #3718

Description

Following ext-skills #157, two gaps remain in the Python SDK:

• resources/directory/read results omit ttlMs and cacheScope, which are now required by the updated specification.
• The Skill model does not support optional _meta; metadata is discarded during parsing and serialization.

Expected fixes

• Add CacheableResult support to directory-read results and apply the existing protocol-version-aware handling: include cache fields for 2026-07-28 and later, and omit them for older connections.
• Add an optional meta field to Skill, serialized as _meta, and preserve it through list/get operations.
• Add regression tests and update the relevant documentation and examples.

References

• Original SDK implementation: Add Skills extension #3485
• Specification correction: Align skills spec types and examples with the 2026-07-28 base schema ext-skills#157
• Original schema inconsistency report: skills.mdx: ReadResourceDirectoryRequest does not type-check against the 2026-07-28 base schema ext-skills#156
• Skills conformance implementation: feat(sep-2640): skills server conformance against the Accepted SEP, enumeration + manifest + directory conformance#330

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