Plan independent BFF and adapt selected Sendico foundations (#34)

backend/AGENTS.md

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+# BFF guidelines
+
+Follow root AGENTS.md. This is an independent Go module; Sendico is an external
+provider, not a local module dependency. Never add sibling-path replacements,
+shared database access or internal Sendico discovery/messaging dependencies.
+
+Keep HTTP DTOs/mappers, application models/use cases and provider adapters distinct.
+Resolve ownership from the authenticated SXCHNGE principal and server-side mappings.
+Only adapters may expose provider transport types internally. Adapted code belongs
+to SXCHNGE; record source revision and changes in PROVENANCE.md.
+
+Review the complete backend contribution before delivery. Run `make test-backend`
+and `make lint-backend` at the repository root. Add behavioral tests for changed
+boundaries and failure paths. Errors returned publicly must be stable typed codes;
+never expose raw provider errors, secrets, document contents or credentials.