[codex] Add Excalidraw skill

[furukama]

What changed

• add a bundled excalidraw skill for editable .excalidraw diagram generation
• add supporting reference docs and agents/openai.yaml
• add a native Node upload helper that creates Excalidraw share links without a Python dependency
• update bundled skill docs to reflect 34 shipped skills and mention excalidraw

Why

HybridClaw already ships workflow skills for generated assets such as Manim. This adds a lightweight diagramming path for architecture diagrams, flowcharts, sequence diagrams, and concept maps, based on the Hermes Excalidraw skill but adapted to HybridClaw's skill conventions and Node-first runtime.

Impact

Users can now ask HybridClaw for editable Excalidraw diagrams and optionally generate a shareable Excalidraw URL directly from the bundled helper.

Validation

• python3 skills/skill-creator/scripts/quick_validate.py skills/excalidraw
• node --import tsx -e "import { loadSkillCatalog } from './src/skills/skills.ts'; const catalog = loadSkillCatalog(); const skill = catalog.find((entry) => entry.name === 'excalidraw'); console.log(JSON.stringify({count: catalog.length, skill}, null, 2));"
• node skills/excalidraw/scripts/upload.mjs /tmp/hybridclaw-excalidraw-test.excalidraw