Add Image Pool (Grid) node design doc

Approved design for an input-side ComfyUI node holding a curated pool of
images with per-image masks and labels, selectable for output. Captures
IO, managed-pool-folder storage, MaskEditor reuse, server routes, edge
cases, and a 3-phase build plan.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,8 @@
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+*.egg-info/
+.pytest_cache/
+.DS_Store
+input/grid_pool/