docs: document the webjs MCP server and check --json on the ai-first docs page

[vivek7405]

Problem

The read-only webjs mcp server and its knowledge / source layer (#376, #378) are documented in AGENTS.md and the scaffold templates, but the docs site has zero coverage. The AI-first page (docs/app/docs/ai-first/page.ts), the natural home, never mentions webjs mcp at all, and no docs-site page documents the MCP server or its init / docs / source tools and resources / prompts knowledge layer. The agent-facing webjs check --json flag (#262) is also absent from the docs site (it is in the AGENTS.md CLI reference only).

Implementation notes (for the implementing agent)

• Where to edit: docs/app/docs/ai-first/page.ts. Add a "webjs MCP server" section after the AGENTS.md section.
• What to document: npx @webjsdev/mcp (also reachable as webjs mcp), the introspection tools (list_routes, list_actions, list_components, check), and the knowledge / authoring layer (init, docs, source tools plus resources / prompts). Mention webjs check --json for an agent loop.
• Source of truth to copy from: packages/cli/templates/AGENTS.md (the "Use the webjs MCP server" section, around L100 to L140), root AGENTS.md around L109, and the CLI reference line for webjs check --json.

Acceptance criteria

• The ai-first docs page documents the webjs mcp server, its introspection tools, and the knowledge / source layer.
• The ai-first (or conventions) docs page mentions webjs check --json.