Add diagramming skills for Excalidraw

- Introduced a new DESCRIPTION.md file outlining diagram creation skills for visual diagrams and flowcharts using Excalidraw.
- Added SKILL.md for the Excalidraw skill, detailing its functionality, usage, and workflow for creating hand-drawn style diagrams.
- Created references for color palettes, dark mode diagrams, and example diagrams to assist users in utilizing the Excalidraw skill effectively.
- Implemented an upload script for sharing diagrams via Excalidraw.com, ensuring user-friendly access to generated diagrams.

skills/diagramming/excalidraw/references/dark-mode.md

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+# Excalidraw Dark Mode Diagrams
+
+To create a dark-themed diagram, use a massive dark background rectangle as the **first element** in the array. Make it large enough to cover any viewport:
+
+```json
+{
+  "type": "rectangle", "id": "darkbg",
+  "x": -4000, "y": -3000, "width": 10000, "height": 7500,
+  "backgroundColor": "#1e1e2e", "fillStyle": "solid",
+  "strokeColor": "transparent", "strokeWidth": 0
+}
+```
+
+Then use the following color palettes for elements on the dark background.
+
+## Text Colors (on dark)
+
+| Color | Hex | Use |
+|-------|-----|-----|
+| White | `#e5e5e5` | Primary text, titles |
+| Muted | `#a0a0a0` | Secondary text, annotations |
+| NEVER | `#555` or darker | Invisible on dark bg! |
+
+## Shape Fills (on dark)
+
+| Color | Hex | Good For |
+|-------|-----|----------|
+| Dark Blue | `#1e3a5f` | Primary nodes |
+| Dark Green | `#1a4d2e` | Success, output |
+| Dark Purple | `#2d1b69` | Processing, special |
+| Dark Orange | `#5c3d1a` | Warning, pending |
+| Dark Red | `#5c1a1a` | Error, critical |
+| Dark Teal | `#1a4d4d` | Storage, data |
+
+## Stroke and Arrow Colors (on dark)
+
+Use the standard Primary Colors from the main color palette -- they're bright enough on dark backgrounds:
+- Blue `#4a9eed`, Amber `#f59e0b`, Green `#22c55e`, Red `#ef4444`, Purple `#8b5cf6`
+
+For subtle shape borders, use `#555555`.
+
+## Example: Dark mode rectangle
+
+```json
+{
+  "type": "rectangle", "id": "r1",
+  "x": 100, "y": 100, "width": 200, "height": 80,
+  "backgroundColor": "#1e3a5f", "fillStyle": "solid",
+  "strokeColor": "#4a9eed", "strokeWidth": 2,
+  "roundness": { "type": 3 },
+  "label": { "text": "Dark Node", "fontSize": 20 }
+}
+```
+
+Note: the label text color will default to `#1e1e1e` which is invisible on dark. If using labels on dark-filled shapes, the Excalidraw renderer handles text color automatically based on the background. For standalone text elements, explicitly set `"strokeColor": "#e5e5e5"`.