v0.20.0 — the bridge leaves the system prompt alone

When a server sends no system_prompt, the bridge no longer substitutes its one-sentence stand-in ("You are an AI assistant… reply in plain prose") in isolated and workspace. The CLI keeps its own default prompt instead; the lifecycle addendum (bridge_prompt) is unchanged.

Behaviour change: a server that relied on the stand-in and sends no system_prompt now gets the CLI's default prompt. Send your own system_prompt to set the assistant's voice.