Starts a named agent session (Claude Code, Codex, pi, or any Herdr-supported kind) in a new Herdr tab in the current workspace, optionally on a specific model. Use when the user asks to spawn an agent, open a new tab running Claude, Codex, or pi, or hand a side task to a fresh session (e.g. "/spawn-agent ABC-123", "spin up a codex agent for the docs"). Do NOT use to implement a ticket (use implement-ticket). Requires HERDR_ENV=1.
