refactor(rag_engine): obliterate legacy _chunk_code wrapper (Phase 5)

Phase 5 (1 of 9 cruft sites obliterated):

OBLITERATED: RAGEngine._chunk_code wrapper. It delegated to _chunk_code_result
and provided a fallback to _chunk_text on AST failure.

Migration: index_file() now calls _chunk_code_result directly with .ok check
+ chunk-size threshold check + fallback to _chunk_text inline. The structured
ErrorInfo is propagated if needed (no caller currently consumes it).

Sub-track 5 tests updated:
- tests/tier2/phase13_invariant_test.py: _chunk_code moved to obliterated list
- tests/tier2/phase13_site2_test.py: _legacy_no_broad_except -> _legacy_obliterated
- tests/test_cruft_removal.py: 2 new tests (wrapper-obliterated invariant +
  caller-uses-result invariant)

PITFALL encountered: the edit_file tool removed a leading space on the
next class method's 'def' line, causing an IndentationError. Fixed by
binary-write replacement preserving CRLF + leading-space styleguide convention
(project uses 1-space indentation; class body methods start at column 1).

Test result: 124/124 pass.
Audit gate: src/rag_engine.py --strict exits 0 (no new violations).
Wrapper count: 3 -> 2 (Phase 6 remaining: gui_2 2).

src/rag_engine.py

@@ -247,16 +247,6 @@ class RAGEngine:
    )
 
 
- def _chunk_code(self, content: str, file_path: str) -> List[str]:
-  """AST-aware chunking for Python code."""
-  ast_result = self._chunk_code_result(content, file_path)
-  if not ast_result.ok:
-   return self._chunk_text(content)
-  chunks = ast_result.data
-  if not chunks or len(content) < self.config.chunk_size:
-   return self._chunk_text(content)
-  return chunks
-
  def _get_file_mtime_result(self, full_path: str) -> Result[float]:
   """Get file modification time. Returns Result[float]."""
   try:
@@ -331,7 +321,11 @@ class RAGEngine:
   self.collection.delete(where={"path": file_path})
 
   if file_path.lower().endswith(".py"):
-   chunks = self._chunk_code(content, file_path)
+   chunk_result = self._chunk_code_result(content, file_path)
+   if not chunk_result.ok or not chunk_result.data or len(content) < self.config.chunk_size:
+    chunks = self._chunk_text(content)
+   else:
+    chunks = chunk_result.data
   else:
    chunks = self._chunk_text(content)