Add experimental MCP skill publication - #4

Summary

Adds an experimental MCP/SEP-2640 publication mode alongside the existing Agent Skills well-known discovery output.

With skills({ mcp: true }) or the object form, Astro sites can publish:

• /.well-known/mcp/skills/index.json with SEP-style frontmatter, url, digest, and optional archives[]
• direct static routes for every file in each skill directory
• generated .tar.gz archive alternatives for multi-file skills
• a .tree.json helper manifest that static MCP servers can use to implement resources/directory/read

The existing /.well-known/agent-skills/index.json output remains enabled by default, so a site can support both the Agent Skills well-known discovery proposal and MCP resource publication.

Related work

• MCP SEP-2640: SEP-2640: Skills Extension modelcontextprotocol/modelcontextprotocol#2640
• Agent Skills well-known discovery proposal: Add spec for .well-known URI agentskills/agentskills#254
• Cloudflare docs bridge PR: [Agents] Publish MCP skill artifacts cloudflare/cloudflare-docs#31606
• Cloudflare MCP server implementation: Add skills resources to Cloudflare API code mode server cloudflare/mcp#165

Validation

• pnpm build
• pnpm build in example/
• Verified generated Agent Skills discovery digests against emitted artifacts
• Verified generated MCP index digests against emitted SKILL.md resources
• Verified archive layout has SKILL.md at the archive root