Updates the skills documentation to include the missing `vercel-sandbox` skill that was missing from both the available skills list and installation commands.
## Changes
- Added `vercel-sandbox` skill to the Available Skills list with description
- Added installation command for `vercel-sandbox` skill
- Added dedicated section for `vercel-sandbox` with key features and usage details
- Removed duplicate paragraph in the electron section
The `vercel-sandbox` skill enables running agent-browser + headless Chrome inside ephemeral Vercel Sandbox microVMs with features like snapshot startup, persistent workflows, and automatic OIDC authentication.
Fixes#712
Chris Tate
d678058206