fix: prevent ghost slots when codex mcp-server spawns multiagents-peer

codex mcp-server loads multiagents-peer from ~/.codex/config.toml and
spawns it as a child MCP server. That adapter auto-registers with the
broker, creating duplicate unnamed slots (ghost agents in dashboard).

Fix: set MULTIAGENTS_DRIVER_MODE=1 env var when spawning via CodexDriver.
The base adapter skips broker registration + poll/heartbeat loops when
this flag is set, since the CodexDriver manages all communication.