Terminology Rename, and named-graphs for explainability (#682)

Terminology Rename, and named-graphs for explainability data

Changed terminology:
  - session -> question
  - retrieval -> exploration
  - selection -> focus
  - answer -> synthesis

- uris.py: Renamed query_session_uri → question_uri,
  retrieval_uri → exploration_uri, selection_uri → focus_uri,
  answer_uri → synthesis_uri
- triples.py: Renamed corresponding triple generation functions with
  updated labels ("GraphRAG question", "Exploration", "Focus",
  "Synthesis")
- namespaces.py: Added named graph constants GRAPH_DEFAULT,
  GRAPH_SOURCE, GRAPH_RETRIEVAL
- init.py: Updated exports
- graph_rag.py: Updated to use new terminology
- invoke_graph_rag.py: Updated CLI to display new stage names
  (Question, Exploration, Focus, Synthesis)

Query-Time Explainability → Named Graph
- triples.py: Added set_graph() helper function to set named graph
  on triples
- graph_rag.py: All explainability triples now use GRAPH_RETRIEVAL
  named graph
- rag.py: Explainability triples stored in user's collection (not
  separate collection) with named graph

Extraction Provenance → Named Graph
- relationships/extract.py: Provenance triples use GRAPH_SOURCE
  named graph
- definitions/extract.py: Provenance triples use GRAPH_SOURCE
  named graph
- chunker.py: Provenance triples use GRAPH_SOURCE named graph
- pdf_decoder.py: Provenance triples use GRAPH_SOURCE named graph

CLI Updates
- show_graph.py: Added -g/--graph option to filter by named graph and
  --show-graph to display graph column

Also:
- Fix knowledge core schemas

trustgraph-base/trustgraph/schema/services/retrieval.py

@@ -22,8 +22,8 @@ class GraphRagResponse:
     error: Error | None = None
     response: str = ""
     end_of_stream: bool = False       # LLM response stream complete
-    explain_id: str | None = None  # Single explain URI (announced as created)
-    explain_collection: str | None = None  # Collection where explain was stored
+    explain_id: str | None = None     # Single explain URI (announced as created)
+    explain_graph: str | None = None  # Named graph where explain was stored (e.g., urn:graph:retrieval)
     message_type: str = ""            # "chunk" or "explain"
     end_of_session: bool = False      # Entire session complete