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/test_engine_routing.py

@@ -64,6 +64,18 @@ def test_content_has_local_images():
     assert notion_writer._content_has_local_images("no images") is False
 
 
+def test_local_image_inside_fence_ignored():
+    """A local image reference inside a fenced code block must NOT be detected
+    as a real image by _content_has_local_images or extracted by extract_local_images."""
+    fenced = "```markdown\n![x](./y.png)\n```"
+    assert notion_writer._content_has_local_images(fenced) is False, \
+        "_content_has_local_images should return False for image inside fence"
+    without, imgs = notion_writer.extract_local_images(fenced)
+    assert len(imgs) == 0, "extract_local_images should not extract image inside fence"
+    # The fenced lines (including the image line) should be preserved verbatim
+    assert "![x](./y.png)" in without, "fence content must be preserved in output"
+
+
 def run_all():
     tests = [
         test_create_page_markdown,
@@ -71,6 +83,7 @@ def run_all():
         test_replace_markdown,
         test_extract_local_images_splits,
         test_content_has_local_images,
+        test_local_image_inside_fence_ignored,
     ]
     for t in tests:
         print(f"\n{t.__name__}")