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

tests/integration/test_agent_kg_extraction_integration.py

@@ -128,7 +128,7 @@ class TestAgentKgExtractionIntegration:
             
             # Parse and process
             extraction_data = extractor.parse_jsonl(agent_response)
-            triples, entity_contexts = extractor.process_extraction_data(extraction_data, v.metadata)
+            triples, entity_contexts, extracted_triples = extractor.process_extraction_data(extraction_data, v.metadata)
 
             # Emit outputs
             if triples: