Deliver explainability triples inline in retrieval response stream (#763)

Provenance triples are now included directly in explain messages from
GraphRAG, DocumentRAG, and Agent services, eliminating the need for
follow-up knowledge graph queries to retrieve explainability details.

Each explain message in the response stream now carries:
- explain_id: root URI for this provenance step (unchanged)
- explain_graph: named graph where triples are stored (unchanged)
- explain_triples: the actual provenance triples for this step (new)

Changes across the stack:
- Schema: added explain_triples field to GraphRagResponse,
  DocumentRagResponse, and AgentResponse
- Services: all explain message call sites pass triples through
  (graph_rag, document_rag, agent react, agent orchestrator)
- Translators: encode explain_triples via TripleTranslator for
  gateway wire format
- Python SDK: ProvenanceEvent now includes parsed ExplainEntity
  and raw triples; expanded event_type detection
- CLI: invoke_graph_rag, invoke_agent, invoke_document_rag use
  inline entity when available, fall back to graph query
- Tech specs updated

Additional explainability test

trustgraph-cli/trustgraph/cli/invoke_agent.py

@@ -182,16 +182,18 @@ def question_explainable(
                 print(item.content, end="", flush=True)
 
             elif isinstance(item, ProvenanceEvent):
-                # Process provenance event immediately
+                # Use inline entity if available, otherwise fetch from graph
                 prov_id = item.explain_id
                 explain_graph = item.explain_graph or "urn:graph:retrieval"
 
-                entity = explain_client.fetch_entity(
-                    prov_id,
-                    graph=explain_graph,
-                    user=user,
-                    collection=collection
-                )
+                entity = item.entity
+                if entity is None:
+                    entity = explain_client.fetch_entity(
+                        prov_id,
+                        graph=explain_graph,
+                        user=user,
+                        collection=collection
+                    )
 
                 if entity is None:
                     if debug:

trustgraph-cli/trustgraph/cli/invoke_document_rag.py

@@ -45,16 +45,18 @@ def question_explainable(
                 print(item.content, end="", flush=True)
 
             elif isinstance(item, ProvenanceEvent):
-                # Process provenance event immediately
+                # Use inline entity if available, otherwise fetch from graph
                 prov_id = item.explain_id
                 explain_graph = item.explain_graph or "urn:graph:retrieval"
 
-                entity = explain_client.fetch_entity(
-                    prov_id,
-                    graph=explain_graph,
-                    user=user,
-                    collection=collection
-                )
+                entity = item.entity
+                if entity is None:
+                    entity = explain_client.fetch_entity(
+                        prov_id,
+                        graph=explain_graph,
+                        user=user,
+                        collection=collection
+                    )
 
                 if entity is None:
                     if debug:

trustgraph-cli/trustgraph/cli/invoke_graph_rag.py

@@ -667,16 +667,18 @@ def _question_explainable_api(
                 print(item.content, end="", flush=True)
 
             elif isinstance(item, ProvenanceEvent):
-                # Process provenance event immediately
+                # Use inline entity if available, otherwise fetch from graph
                 prov_id = item.explain_id
                 explain_graph = item.explain_graph or "urn:graph:retrieval"
 
-                entity = explain_client.fetch_entity(
-                    prov_id,
-                    graph=explain_graph,
-                    user=user,
-                    collection=collection
-                )
+                entity = item.entity
+                if entity is None:
+                    entity = explain_client.fetch_entity(
+                        prov_id,
+                        graph=explain_graph,
+                        user=user,
+                        collection=collection
+                    )
 
                 if entity is None:
                     if debug: