Pair category repair (#550) and saved instructions + dialog picker (#431) (#590)

* feat(maintenance): repair old Pair prompts' empty category and tags (#550)

Squashed from PR #579. Closes #550.

* feat(tagging): saved instructions with a default per engine (#431)

Squashed from PR #587.

* feat(tagging): pick saved instructions in the tagging dialogs (#431)

Squashed from PR #588. Closes #431.

tests/test_api_annotations.py

@@ -1144,8 +1144,10 @@ class TestEnvelopes(AnnotationAPITestCase):
         self.assertEqual(
             sorted(await self.json_of(resp)),
             [
+                "default_instructions",  # #431 adds the saved instructions
                 "default_prompt",
                 "existing",  # #328 adds the saved batch policy and model
+                "instructions",
                 "loaded_model_type",
                 "model_loaded",
                 "model_type",