Give every agent in Herdr its own Vercel Sandbox

Herdr now supports running terminal coding agents like Claude Code, Codex, and OpenCode in isolated Vercel Sandboxes, allowing multiple agents to operate concurrently without affecting the local machine. The plugin uploads project files to a sandbox, runs the agent there, and provides a terminal-like pane for interaction, with all actions machine-readable and safeguards to control file changes.