docs: define v0.1 product and architecture contract

docs/adr/0001-version-the-json-contract.md

@@ -0,0 +1,16 @@
+---
+status: accepted
+---
+
+# Version the JSON contract by compatibility boundary
+
+Automation agents are the primary interface consumers, so success and error
+shapes must remain predictable across upgrades. The project publishes reviewed
+JSON Schema files under `schemas/v1/`: optional fields may be added within v1
+and consumers must ignore unknown fields, while removing a field or changing
+its type or meaning requires a new major schema version. The CLI initially
+emits only its current schema rather than carrying multiple encoders; golden
+fixtures and schema validation gate every success and error change. JSON mode
+also preserves strict stream framing: a success is the only stdout document, a
+failure is the only stderr document, and progress or diagnostic logs never
+share either stream.