Add public repository interfaces and subclassable adapter CRUD API.

Define split JSON and object repository ABCs, promote adapter methods to public CRUD, add example domain repositories, modernize interface stubs, and make the package installable for tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

python_repositories/examples/__init__.py

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+"""Example domain repositories built on technology adapters."""

python_repositories/examples/artifact_object_repository.py

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+"""Example domain repository backed by MinIO objects."""
+
+from io import BytesIO
+
+from python_repositories.adapters.minio_adapter import MinioAdapter
+
+
+class ArtifactObjectRepository(MinioAdapter):
+    """Example: domain repository backed by MinIO objects."""
+
+    def _object_name(self, artifact_id: str) -> str:
+        return f"artifacts/{artifact_id}"
+
+    def get_artifact(self, artifact_id: str) -> BytesIO | None:
+        return self.get(self._object_name(artifact_id))
+
+    def store_artifact(
+        self,
+        artifact_id: str,
+        data: BytesIO,
+        content_type: str = "application/octet-stream",
+    ) -> None:
+        self.put(self._object_name(artifact_id), data, content_type)

python_repositories/examples/user_json_repository.py

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+"""Example domain repository backed by Redis JSON."""
+
+from python_repositories.adapters.redis_adapter import RedisAdapter
+
+
+class UserJsonRepository(RedisAdapter):
+    """Example: domain repository backed by Redis JSON."""
+
+    def _key(self, user_id: str) -> str:
+        return f"user:{user_id}"
+
+    def get_user(self, user_id: str) -> dict | None:
+        return self.get(self._key(user_id))
+
+    def save_user(self, user_id: str, user: dict) -> None:
+        self.set(self._key(user_id), user)
+
+    def delete_user(self, user_id: str) -> None:
+        self.delete(self._key(user_id))