PRD: engagement-mode decision branch in the message machine

[cgrpa]

Parent

cgrpa/TheSexy6BotWorker#41

What to build

Add the engagement-only decision stage to the per-message machine. After tool execution has fully settled, the machine should decide whether the bot should reply at all for an engagement-mode message.

The decision stage should only apply to engagement-mode turns, and it should be the only place where the turn can decide to produce no response.

Acceptance criteria

• Engagement-mode turns can reach a decision stage after tool execution completes.
• The decision stage can choose to respond or not respond.
• Direct-prefix turns do not pass through the engagement decision stage.
• The decision is based on the final settled turn context.

Blocked by

• cgrpa/TheSexy6BotWorker#43
• cgrpa/TheSexy6BotWorker#44