Add MCP server endpoint at /mcp

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Summary

Adds an MCP (Model Context Protocol) endpoint at /mcp so LLM-based agents can discover and query the identity API via the standard MCP Streamable HTTP transport.

What's new

• /mcp endpoint — Serves MCP tools backed by the existing GraphQL API. Executes queries internally (no network hop) with full dataloader support.
• @mcpTool directive — Annotates commonly used queries in the GraphQL schema. The codegen tool reads these to generate typed shortcut tools.
• No auth required — identity-api is a public read API, so the MCP handler is mounted without auth middleware (same as /query).

MCP tools exposed

Tool	Description
identity_get_schema	Returns the GraphQL schema via introspection
identity_query	Executes arbitrary GraphQL queries
identity_get_vehicle	Look up a vehicle by token ID or DID
identity_list_vehicles	List vehicles with optional filters

Dependencies

• Requires server-garage#25 (pkg/mcpserver + cmd/mcpgen)
• Uses go mod replace for local development — will be updated to a tagged version after server-garage is merged

Files changed

• graph/schema/directives.graphqls — @mcpTool directive definition
• graph/schema/vehicle.graphqls — @mcpTool annotations on vehicle and vehicles queries
• graph/mcp_tools_gen.go — Generated tool definitions (via mcpgen)
• graph/resolver.go — Added //go:generate mcpgen directive
• gqlgen.yml — mcpTool: skip_runtime: true
• cmd/identity-api/main.go — Mount MCP handler at /mcp

Test plan

• Builds cleanly
• Existing tests unaffected
• Smoke test /mcp endpoint locally with MCP client
• Verify identity_get_schema returns introspection data
• Verify identity_get_vehicle returns vehicle data