refactor: solidify startup UX and engine-aware preprocessing

- Slim core deps: move ML stack to optional extras (paddle/tables/figures/scientific/full)
- Lazy settings proxy with config search paths (env var, cwd, user dir)
- New commands: init, demo, setup [basic|full], first-run guard on run/watch
- Engine-aware preprocessing: Paddle gets original image (fixes dark mode 0.83->0.95)
- Results table shows Skipped count; lazy run-dir creation
- kg_ocr marked experimental with extra, Docker defaults with OCR_PIPELINE_CONFIG
- 25/25 tests, ruff clean

kg_ocr/__init__.py

@@ -1,5 +1,17 @@
-from .ocr import get_screenshots, extract_text
-from .embeddings import create_and_index
-from .rag import retrieve, ask_wllm
+"""Experimental txtai/litellm RAG interface built on the OCR pipeline.
+
+This package is a prototype. The supported interface is the `ocr-pipeline` CLI
+(src/ocr_pipeline). Extra dependencies are required: `uv sync --extra kg`.
+"""
+
+try:
+    from .ocr import get_screenshots, extract_text
+    from .embeddings import create_and_index
+    from .rag import retrieve, ask_wllm
+except ImportError as exc:
+    raise ImportError(
+        "kg_ocr is experimental and needs extra dependencies: "
+        "uv sync --extra kg  (or: uv pip install txtai litellm python-dotenv)"
+    ) from exc
 
 __all__ = ["get_screenshots", "extract_text", "create_and_index", "retrieve", "ask_wllm"]