feat: add GraphQL SDL importer (graphqlerd.py) (#121)

Parses GraphQL SDL (.graphql / .gql) or an introspection JSON dump into an
entity type diagram: one node per type, interface, input, enum, union or
custom scalar, listing each field with its type and marking @deprecated,
with edges for field references, implements and union membership.

The parser is stdlib-only. Enums and custom scalars are dimmed rather than
unlinked, and --group buckets types by the schema file they came from.

Registered in SKILL.md, toolbox.md, cookbook.md, diagram-ir.md, mcp.md, both
READMEs and the CHANGELOG, and wired into diagramctl build and the MCP server
with .graphql/.gql auto-detection.