feat(ensapi): introduce Model Context Protocol (MCP) server for Omnigraph API

[djstrong]

Lite PR

Tip: Review docs on the ENSNode PR process

Summary

• Adds a first-party Omnigraph MCP server at /api/mcp on every ENSApi instance (streamable-HTTP), with tools (omnigraph_query, omnigraph_schema), schema/example resources, prompts, and validation hints for common GraphQL mistakes.
• Adds offline schema lookup helpers in @ensnode/ensnode-sdk/internal (lookupOmnigraphSchema, buildCondensedSchemaReference) with bundled SDL so schema loading works in Vite/Rollup builds (no Node createRequire).
• Adds docs (omnigraph-mcp.mdx, AI/LLM integration page link) and refactors enscli / ensskills to share the same schema helpers.

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Why

• AI agents (Cursor, Claude Desktop, etc.) need a standard way to query ENS state without bespoke per-use-case tools or hand-maintained GraphQL.
• MCP gives agents execution against the full Omnigraph; bundled schema lookup + vetted example queries reduce bad first attempts (wrong field names, camelCase filters, subgraph-style patterns).
• Consolidates schema reference logic in ensnode-sdk so ENSApi MCP, enscli, and ensskills stay in sync.

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Testing

• Manually tested against Cursor/Claude Desktop; docs cover client setup.

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Notes for Reviewer (Optional)

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Pre-Review Checklist (Blocking)

• This PR does not introduce significant changes and is low-risk to review quickly.
• Relevant changesets are included (or are not required)