docs(ontex): align OpenAPI guidance with aggregate strategy
openapi-first now defaults to the aggregate strategy (one docs/api/openapi.yaml generating into com.ontex.api). Update the agent guidelines + conventions to match what the build actually does, so a coding agent edits the right file:
- AGENTS.md rule 4 + the "where to put code" table → new endpoints are an entry in docs/api/openapi.yaml; controllers implement the generated com.ontex.api.Api. Keep each module's paths under a //… prefix, tagged with the module.
- conventions.md OpenAPI section + the require-openapi-spec gate note → the aggregate openapi.yaml.