Studio RAG trim: remove cross-encoder reranker

The cross-encoder reranker is off by default (enable_rerank=False everywhere)
and adds a second model download plus a candidate-widening pass on every search.
Removing it keeps the core retrieval (parse, chunk, embed, BM25 + dense, RRF,
search_knowledge_base tool) intact while dropping ~375 lines.

- delete core/rag/reranker.py and its test
- drop enable_rerank / reranker_model from the search tool, tools dispatch,
  and the /rag/search route (candidate_k is now just top_k)
- remove the /rag/reranker/precache endpoint and reranker config knobs
- update tool-handler test to the trimmed rag_scope shape

42 RAG tests pass.

tests/python/test_rag_tool_handler.py

@@ -196,8 +196,6 @@ def test_execute_tool_dispatches_to_search_knowledge_base():
         top_k = None,
         scope_kb_id = None,
         scope_thread_id = None,
-        enable_rerank = False,
-        reranker_model = None,
         default_top_k = 5,
         min_score = 0.0,
         **kwargs,
@@ -206,7 +204,6 @@ def test_execute_tool_dispatches_to_search_knowledge_base():
         called["top_k"] = top_k
         called["scope_kb_id"] = scope_kb_id
         called["scope_thread_id"] = scope_thread_id
-        called["enable_rerank"] = enable_rerank
         called["default_top_k"] = default_top_k
         called["min_score"] = min_score
         return "stub-result"
@@ -218,7 +215,6 @@ def test_execute_tool_dispatches_to_search_knowledge_base():
             tool_context = {
                 "rag_scope": {
                     "kb_id": "kb-1",
-                    "enable_rerank": True,
                     "default_top_k": 3,
                     "min_score": 0.35,
                 }
@@ -229,7 +225,6 @@ def test_execute_tool_dispatches_to_search_knowledge_base():
     assert called["top_k"] == 7
     assert called["scope_kb_id"] == "kb-1"
     assert called["scope_thread_id"] is None
-    assert called["enable_rerank"] is True
     assert called["default_top_k"] == 3
     assert called["min_score"] == 0.35