PRD: bounded tool execution loop for per-message turns

[cgrpa]

Parent

cgrpa/TheSexy6BotWorker#41

What to build

Add an explicit tool-execution stage inside the per-message machine. This slice should let the machine invoke function calling with a bounded continuation loop, so model/tool interactions can settle before later rendering or decision steps.

The tool-execution stage should mutate the mutable per-turn context, support the existing manual continuation behavior, and terminate cleanly on tool errors instead of leaving the turn in an ambiguous state.

Acceptance criteria

• The machine can execute tool calls in a bounded loop.
• Tool results are appended to the per-turn context as the turn progresses.
• Tool execution failures terminate the turn cleanly.
• The tool stage is usable by the direct-prefix path and is ready for the engagement path.

Blocked by

• cgrpa/TheSexy6BotWorker#42
• cgrpa/TheSexy6BotWorker#43