guides: add AI Integration section with MCP-for-agents page

[deliciousmonster]

Summary

• Adds a new AI Integration section to guides/SUMMARY.md with a single starter page, guides/ai/mcp.md.
• The page is the customer-facing how-to companion to reference/api/operations.md (which is the wire-spec reference): what every `@export`-ed table exposes for free, how to opt out with `@export(mcp: false)`, how to combine with `@access(public: [read])` for read-only agents, and how to wire Claude Desktop / Cursor / Claude Code.
• Wire-level JSON-RPC catalog, session lifecycle, and error codes stay where they belong — in the reference book.

Why

docs/guides/ in the platform repo had four guides not mirrored in yeti-documentation: building.md, development.md, testing.md, and mcp.md. The first three are contributor-facing (build-from-source, workspace layout, platform test harness) and correctly excluded. mcp.md is customer-facing — an app developer needs to know their tables already speak MCP and how to connect a client — so it belongs here. This PR closes that gap with a guide-shaped doc (~200 lines, code-forward) rather than reproducing the 660-line in-repo file verbatim.

Test plan

• `mdbook build` cleanly under `source/books/guides/`
• Sidebar shows new AI Integration section between Real-Time Features and Caching & Performance
• All relative links resolve: `../../reference/api/operations.md`, `../auth/jwt.md`, `../auth/overview.md`, `../caching/rate-limiting.md`, `../../reference/config/schema-directives.md`, `../../reference/api/rest.md`
• Schema snippets render with correct GraphQL highlighting
• curl + JSON snippets render with correct highlighting

🤖 Generated with Claude Code