fix: remove agent from pnpm workspace to fix Render build

The agent's package.json made it a pnpm workspace member, so
pnpm install on the Node frontend service triggered "uv sync" in an
environment where uv doesn't exist — breaking Render deploys.

Remove apps/agent/package.json entirely (matching the Shadify pattern)
and run the agent dev server via a direct shell command in the root
package.json instead of through Turbo.