fix(notion-writer): fence-aware image/translate, NtnUploadError boundary, dead-code removal

FIX 1: _content_has_local_images and extract_local_images now skip lines inside
  ``` fences so a local-image ref inside a code block is not treated as real.
FIX 2: md_translate.translate() tracks fence state; callout/columns/mention
  transforms are suppressed for lines inside ``` ... ``` blocks (pass verbatim).
FIX 3: main() catches NtnUploadError and prints a clean error + e.stderr before
  sys.exit(1); body moved to _main_body(parser, args).
FIX 4: ntn_files.upload() raises NtnUploadError on empty stdout instead of
  IndexError, giving a clean message combined with FIX 3.
FIX 5: Removed unused write_to_page() (both page paths are inlined).
FIX 6: create_page_markdown() omits the 'markdown' key when value is falsy,
  avoiding empty-markdown sends on DB rows created without --file/--stdin.
FIX 7: CLAUDE.md documents --allow-deleting-content scope (markdown engine only).
TDD: Added test_fence_passthrough_no_transform (md_translate, suite now 8) and
  test_local_image_inside_fence_ignored (engine_routing, suite now 6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

custom-skills/32-notion-writer/code/scripts/md_translate.py

@@ -43,9 +43,23 @@ def translate(content: str) -> str:
     lines = content.split("\n")
     out: List[str] = []
     i = 0
+    in_fence = False
     while i < len(lines):
         line = lines[i]
 
+        # Fence tracking: pass through unchanged, toggle state
+        if line.strip().startswith("```"):
+            in_fence = not in_fence
+            out.append(line)
+            i += 1
+            continue
+
+        # Inside fence: pass through verbatim, no transformation
+        if in_fence:
+            out.append(line)
+            i += 1
+            continue
+
         # Callout: > [!TYPE] then contiguous > body lines
         if line.lstrip().startswith(">"):
             body_first = line.lstrip()[1:].strip()