chore: initialize API repository

docs/contract-testing.md

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+# Contract Testing
+
+Fixtures cover HTTP responses, authenticated WebSocket events, errors, permissions, version negotiation, reconnects, and server-owned state transitions. Fixtures are deterministic, synthetic, and secret-free.
+

docs/versioning.md

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+# API Versioning
+
+- Use an explicit major API version.
+- Prefer additive fields and events.
+- Clients must tolerate unknown fields and events.
+- Breaking changes require a new major version, fixtures, migration notes, and coordinated releases.
+- Each release records minimum compatible client and server versions.
+