Modern skills/get results must carry ttlMs and cacheScope (ext-skills stable spec)

Problem

core/mcp/skillsSchemas.ts treats caching attributes on skills/get as optional, because SEP-2640 said whether they are required was "left open". The stable ext-skills specification (specification/stable/skills.mdx, updated upstream 2026-09-16) now settles that question:

GetSkillResult extends CacheableResult, so ttlMs and cacheScope are REQUIRED, as they are on resources/read, and carry the same semantics.

As a result, the Inspector accepts a modern skills/get result that is missing these required fields, so it reports a non-conforming server as clean.

Expected

On modern (2026-07-28+) connections, validate skills/get results against CacheableResult the same way skills/list already is: require ttlMs as a non-negative integer and cacheScope as public or private. Legacy results stay permissive. Update the "left open" comments to match.

Context

Found in Copilot review of #2401.

Start in core/mcp/skillsSchemas.ts and compare modern skills/get validation with the existing skills/list handling of CacheableResult. Check the stable ext-skills specification in specification/stable/skills.mdx and confirm that modern results require non-negative integer ttlMs and public or private cacheScope while legacy results remain permissive. Done means the Inspector rejects non-conforming modern results and the outdated comments are updated.