Skills store: prep work for the MCP Skills extension (SEP-2640)

Context The MCP Skills extension (io.modelcontextprotocol/skills, [SEP-2640]( [overview]( lets an MCP server serve Agent Skills natively. A host lists them at connect time, shows them in its own skill picker next to local skills, asks the user to approve, and injects them like a local skill. Today the PostHog MCP server delivers store skills only through the exec learn tool verb and the skill-* tools. What the extension needs from a server: • Every skill file is a resource at skill://<prefix>/<name>/<file-path>. The final path segment must equal the frontmatter name. • skills/list returns entries of {uri, frontmatter, resources} where resources is [{uri, digest: "sha256:<hex>", size}] (size in bytes) or the string "dynamic". • skills/get returns one entry by URI. • Content is read with plain resources/read, and the bytes must match the listed digest and size exactly. The parsed frontmatter of SKILL.md must match th…