feat: expand curated skill catalog

[kapelame]

Summary

Add four curated Skill packages on top of the Skill Directory core in #236:

• Anthropic MCP Builder
• Anthropic Skill Creator
• Anthropic Internal Communications
• Anthropic Algorithmic Art

Scope

This stacked PR only adds catalog data and vendored Skill package contents.

• Update catalog/skills/catalog.json from 3 to 7 reviewed entries.
• Update the catalog README source table.
• Vendor the complete upstream package for each new Skill, including supporting files and license files.
• Extend the existing loader test to verify all seven entries and package contents load.

It does not change:

• API or OpenAPI definitions.
• Store, database schema, migrations, or blob storage logic.
• Frontend behavior.
• MCP Directory, OAuth, credential binding, or Agent permission logic.

Reuse and Safety

• Reuses the existing embedded Skill Catalog, Capability Import, Capability Version, Blob Store, and idempotency flow from feat: add curated Skill Directory #236.
• Catalog entries point to pinned Anthropic upstream commit b29e7cf65e5cb78a5ac33d582270551bc74a14eb.
• Upstream license files are retained.
• Import stores package content but does not execute scripts, start processes, bind Agents, or require credentials.
• A repository scan found no API keys, tokens, or private keys.

Verification

• go test ./server/internal/skillcatalog ./server/internal/api/skilldirectory
• make check

Both passed locally.

Base this PR on feat: add curated Skill Directory (#236); merge #236 first, then this catalog expansion.