Studio: tighten RAG code comments

Shorten and condense comments across the RAG backend, frontend, and
tests for readability. Comment text only; no code, strings, identifiers,
or logic changed. License headers and lint/type pragmas are preserved.

tests/python/test_rag_bm25.py

@@ -18,7 +18,7 @@ def isolated_bm25_root(tmp_path, monkeypatch):
     from utils.paths import storage_roots
 
     monkeypatch.setenv("UNSLOTH_STUDIO_HOME", str(tmp_path))
-    # Reset module-level cache between tests.
+    # Reset module cache between tests.
     from core.rag import bm25
 
     bm25._cache.clear()