Subgraph provenance (#694)

Replace per-triple provenance reification with subgraph model

Extraction provenance previously created a full reification (statement
URI, activity, agent) for every single extracted triple, producing ~13
provenance triples per knowledge triple.  Since each chunk is processed
by a single LLM call, this was both redundant and semantically
inaccurate.

Now one subgraph object is created per chunk extraction, with
tg:contains linking to each extracted triple.  For 20 extractions from
a chunk this reduces provenance from ~260 triples to ~33.

- Rename tg:reifies -> tg:contains, stmt_uri -> subgraph_uri
- Replace triple_provenance_triples() with subgraph_provenance_triples()
- Refactor kg-extract-definitions and kg-extract-relationships to
  generate provenance once per chunk instead of per triple
- Add subgraph provenance to kg-extract-ontology and kg-extract-agent
  (previously had none)
- Update CLI tools and tech specs to match

Also rename tg-show-document-hierarchy to tg-show-extraction-provenance.

Added extra typing for extraction provenance, fixed extraction prov CLI

trustgraph-cli/trustgraph/cli/invoke_graph_rag.py

@@ -36,7 +36,7 @@ TG_SELECTED_EDGE = TG + "selectedEdge"
 TG_EDGE = TG + "edge"
 TG_REASONING = TG + "reasoning"
 TG_CONTENT = TG + "content"
-TG_REIFIES = TG + "reifies"
+TG_CONTAINS = TG + "contains"
 PROV = "http://www.w3.org/ns/prov#"
 PROV_STARTED_AT_TIME = PROV + "startedAtTime"
 PROV_WAS_DERIVED_FROM = PROV + "wasDerivedFrom"
@@ -185,18 +185,18 @@ async def _query_edge_provenance(ws_url, flow_id, edge_s, edge_p, edge_o, user,
     """
     Query for provenance of an edge (s, p, o) in the knowledge graph.
 
-    Finds statements that reify the edge via tg:reifies, then follows
+    Finds subgraphs that contain the edge via tg:contains, then follows
     prov:wasDerivedFrom to find source documents.
 
     Returns list of source URIs (chunks, pages, documents).
     """
-    # Query for statements that reify this edge: ?stmt tg:reifies <<s p o>>
+    # Query for subgraphs that contain this edge: ?subgraph tg:contains <<s p o>>
     request = {
         "id": "edge-prov-request",
         "service": "triples",
         "flow": flow_id,
         "request": {
-            "p": {"t": "i", "i": TG_REIFIES},
+            "p": {"t": "i", "i": TG_CONTAINS},
             "o": {
                 "t": "t",  # Quoted triple type
                 "tr": {