Add configurable slides path and malformed marker detection

- project.json now supports "slides" field pointing to slides.json location
- Slide images are loaded from same directory as slides.json
- Validation detects malformed markers (missing ], extra spaces)
- Reports line numbers for each malformed marker

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

gnommo/transformer.py

@@ -37,6 +37,10 @@ def build_render_plan(
     # Build slide events from transcript markers
     slide_events = _extract_slide_events(transcript, slides, total_duration)
 
+    # Derive slides directory from slides_path
+    slides_json_path = project_path / config.slides_path
+    slides_dir = slides_json_path.parent
+
     return RenderPlan(
         project_path=project_path,
         config=config,
@@ -44,6 +48,7 @@ def build_render_plan(
         slide_events=slide_events,
         total_duration=total_duration,
         slides=slides,
+        slides_dir=slides_dir,
     )