docs: add DocGen preview editing demo video, screenshot, and README feature demo section

Author: ijbo

README.md

@@ -340,6 +340,15 @@ Import files directly — they're auto-converted to Markdown client-side:
 
 </details>
 
+<details open>
+<summary><strong>🏷️ DocGen Preview Editing — Editable Prompts & Steps</strong></summary>
+
+**Edit prompts and steps directly in preview cards.** Use `@prompt:` for editable AI instructions (bare text stays as a static description label), edit `@step:` inputs inline in Agent Flow cards, and attach images with 📎 for multimodal vision analysis. All edits sync back to the editor in real-time.
+
+<img src="public/assets/demos/25_docgen_preview_editing.webp" alt="DocGen Preview Editing — editable @prompt: textarea, Agent step inputs, description/prompt separation" width="100%">
+
+</details>
+
 ## 📝 Usage
 
 | Action | How |

assets/docgen-preview-editing.png

@@ -0,0 +1,17 @@
+# CHANGELOG — DocGen Preview Editing Demos
+
+## Summary
+
+Added demo video and screenshot for the DocGen preview editing features.
+
+## Changes
+
+- Added `public/assets/demos/25_docgen_preview_editing.webp` — animated demo showing editable @prompt: textarea, Agent step inputs, and description/prompt separation
+- Added `assets/docgen-preview-editing.png` — screenshot showing all 3 card types
+- Added README Feature Demos section for DocGen Preview Editing
+- Updated README AI Document Tags feature row
+
+## Files Added/Modified
+- `public/assets/demos/25_docgen_preview_editing.webp` [NEW]
+- `assets/docgen-preview-editing.png` [NEW]
+- `README.md` [MODIFIED]