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

docs/tech-specs/query-time-explainability.md

@@ -63,7 +63,11 @@ Explainability events stream to client as the query executes:
 3. Edges selected with reasoning → event emitted
 4. Answer synthesized → event emitted
 
-Client receives `explain_id` and `explain_collection` to fetch full details.
+Client receives `explain_id`, `explain_graph`, and `explain_triples` inline
+in each explain message. The triples contain the full provenance data for
+that step — no follow-up graph query needed. The `explain_id` serves as
+the root entity URI within the triples. Data is also written to the
+knowledge graph for later audit/analysis.
 
 ## URI Structure
 
@@ -144,7 +148,8 @@ class GraphRagResponse:
     response: str = ""
     end_of_stream: bool = False
     explain_id: str | None = None
-    explain_collection: str | None = None
+    explain_graph: str | None = None
+    explain_triples: list[Triple] = field(default_factory=list)
     message_type: str = ""  # "chunk" or "explain"
     end_of_session: bool = False
 ```
@@ -154,7 +159,7 @@ class GraphRagResponse:
 | message_type | Purpose |
 |--------------|---------|
 | `chunk` | Response text (streaming or final) |
-| `explain` | Explainability event with IRI reference |
+| `explain` | Explainability event with inline provenance triples |
 
 ### Session Lifecycle