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/ntn_files.py

@@ -80,6 +80,10 @@ def upload(path: Path) -> str:
             f"ntn upload failed for {path}", path=str(path),
             stderr=result.stderr.strip(),
         )
+    if not result.stdout.strip():
+        raise NtnUploadError(
+            "ntn returned no output", path=str(path), stderr=result.stderr.strip()
+        )
     first_line = result.stdout.strip().splitlines()[0]
     upload_id = first_line.split("\t")[0].strip()
     return upload_id