feat: replace LLM edge scoring with cross-encoder reranker in GraphRAG

Replace the three-prompt LLM scoring pipeline (kg-edge-scoring,
kg-edge-reasoning, kg-edge-selection) with a cross-encoder reranker
service backed by FlashRank. The new hop_and_filter() method performs
iterative graph traversal with semantic scoring at each hop, replacing
the previous follow_edges/get_subgraph approach.

- Add reranker service (trustgraph-base client/service, FlashRank processor)
- Add gateway dispatch for reranker via API and WebSocket
- Rewrite GraphRAG pipeline: hop_and_filter() with per-hop cross-encoder scoring
- Remove kg_prompt() and edge_score_limit from prompt client
- Update provenance: add tg:EdgeSelection type, tg:concept, tg:score predicates
- Update CLIs (tg-invoke-graph-rag, tg-show-explain-trace) for new metadata
- Add tg-invoke-reranker CLI tool
- Add tech spec and UX developer guidance
- Update all unit and integration tests

trustgraph-base/trustgraph/base/__init__.py

@@ -42,6 +42,8 @@ from . dynamic_tool_service import DynamicToolService
 from . tool_service_client import ToolServiceClientSpec
 from . agent_client import AgentClientSpec
 from . structured_query_client import StructuredQueryClientSpec
+from . reranker_client import RerankerClientSpec
+from . reranker_service import RerankerService
 from . row_embeddings_query_client import RowEmbeddingsQueryClientSpec
 from . collection_config_handler import CollectionConfigHandler