Add unified explainability support and librarian storage for (#693)

Add unified explainability support and librarian storage for all retrieval engines

Implements consistent explainability/provenance tracking
across GraphRAG, DocumentRAG, and Agent retrieval
engines. All large content (answers, thoughts, observations)
is now stored in librarian rather than as inline literals in
the knowledge graph.

Explainability API:
- New explainability.py module with entity classes (Question,
  Exploration, Focus, Synthesis, Analysis, Conclusion) and
  ExplainabilityClient
- Quiescence-based eventual consistency handling for trace
  fetching
- Content fetching from librarian with retry logic

CLI updates:
- tg-invoke-graph-rag -x/--explainable flag returns
  explain_id
- tg-invoke-document-rag -x/--explainable flag returns
  explain_id
- tg-invoke-agent -x/--explainable flag returns explain_id
- tg-list-explain-traces uses new explainability API
- tg-show-explain-trace handles all three trace types

Agent provenance:
- Records session, iterations (think/act/observe), and conclusion
- Stores thoughts and observations in librarian with document
  references
- New predicates: tg:thoughtDocument, tg:observationDocument

DocumentRAG provenance:
- Records question, exploration (chunk retrieval), and synthesis
- Stores answers in librarian with document references

Schema changes:
- AgentResponse: added explain_id, explain_graph fields
- RetrievalResponse: added explain_id, explain_graph fields
- agent_iteration_triples: supports thought_document_id,
  observation_document_id

Update tests.

tests/contract/test_translator_completion_flags.py

@@ -110,16 +110,17 @@ class TestRAGTranslatorCompletionFlags:
         assert response_dict["end_of_stream"] is True
         assert response_dict["end_of_session"] is False
 
-    def test_document_rag_translator_is_final_with_end_of_stream_true(self):
+    def test_document_rag_translator_is_final_with_end_of_session_true(self):
         """
         Test that DocumentRagResponseTranslator returns is_final=True
-        when end_of_stream=True.
+        when end_of_session=True.
         """
         # Arrange
         translator = TranslatorRegistry.get_response_translator("document-rag")
         response = DocumentRagResponse(
             response="A document about cats.",
             end_of_stream=True,
+            end_of_session=True,
             error=None
         )
 
@@ -127,9 +128,31 @@ class TestRAGTranslatorCompletionFlags:
         response_dict, is_final = translator.from_response_with_completion(response)
 
         # Assert
-        assert is_final is True, "is_final must be True when end_of_stream=True"
+        assert is_final is True, "is_final must be True when end_of_session=True"
         assert response_dict["response"] == "A document about cats."
+        assert response_dict["end_of_session"] is True
+
+    def test_document_rag_translator_end_of_stream_not_final(self):
+        """
+        Test that end_of_stream=True alone does NOT make is_final=True.
+        The session continues with provenance messages after LLM stream completes.
+        """
+        # Arrange
+        translator = TranslatorRegistry.get_response_translator("document-rag")
+        response = DocumentRagResponse(
+            response="Final chunk",
+            end_of_stream=True,
+            end_of_session=False,  # Session continues with provenance
+            error=None
+        )
+
+        # Act
+        response_dict, is_final = translator.from_response_with_completion(response)
+
+        # Assert
+        assert is_final is False, "end_of_stream=True should NOT make is_final=True"
         assert response_dict["end_of_stream"] is True
+        assert response_dict["end_of_session"] is False
 
     def test_document_rag_translator_is_final_with_end_of_stream_false(self):
         """