Reorganize GraphQL docs: separate Go backend from frontend Relay client
Move frontend Relay client documentation into relay.md and create new graphql.md dedicated to Go backend patterns. Covers gqlgen schema-first approach, @goModel/@goEnum/@goField directives, connection type patterns, and cursor pagination implementation.
- relay.md: Frontend Relay client (environments, compiler, queries, fragments, mutations)
- graphql.md: Go backend gqlgen (directives, connection types, pagination schema, keyset pagination)
- AGENTS.md: Update documentation references
Signed-off-by: Bryan Frimin <bryan@getprobo.com>
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