fix(rag): make dim check robust to file locks (ignore_errors=True)

Replaces self.client.delete_collection(name) with shutil.rmtree on the
collection directory + recreate PersistentClient. This is more robust
to file locks (WinError 32 on Windows) where the live_gui subprocess
holds the file lock on the chroma collection.

The original delete_collection call fails on locked files, leaving the
collection in a broken state (dim mismatch) that causes subsequent
RAG searches to hang. shutil.rmtree with ignore_errors=True handles
this case more gracefully.

Note: This fix is an improvement but may not fully resolve the
test_rag_phase4_final_verify timeout in batched runs. The fundamental
issue is that the live_gui subprocess (session-scoped fixture) holds
file locks on the workspace's .slop_cache, and the test's pre-test
cleanup cannot remove locked files from the same process. A complete
fix would require either changing the fixture scope or implementing
a more sophisticated lock-handling strategy in the RAG engine.

Diagnosis documented in docs/reports/DIAGNOSIS_test_rag_phase4_final_verify.md.

src/rag_engine.py

@@ -15,6 +15,7 @@ from src.result_types       import ErrorInfo, ErrorKind, NilRAGState, Result
 from src.type_aliases       import Metadata
 
 from src.file_cache import ASTParser
+import shutil
 
 
 @dataclass(frozen=True)
@@ -195,7 +196,15 @@ class RAGEngine:
     f"Recreating collection to prevent silent corruption.\n"
    )
    sys.stderr.flush()
-   self.client.delete_collection(self.collection.name)
+   # Per fix_test_rag_phase4_final_verify_diagnosis_20260627: shutil.rmtree with
+   # ignore_errors=True is more robust to file locks (WinError 32 on Windows) where
+   # the live_gui subprocess holds the file lock on the chroma collection. The
+   # original delete_collection call fails on locked files, leaving the collection
+   # in a broken state (dim mismatch) that causes subsequent RAG searches to hang.
+   db_path = os.path.abspath(os.path.join(self.base_dir, ".slop_cache", f"chroma_{self.collection.name}"))
+   if os.path.exists(db_path):
+    shutil.rmtree(db_path, ignore_errors=True)
+   self.client = chromadb.PersistentClient(path=os.path.dirname(db_path))
    self.collection = self.client.get_or_create_collection(name=self.collection.name)
    return Result(data=None)
   except Exception as e: