After injecting the plan as an assistant message into session.context,
the previous code appended a system message saying "Plan is ready.
Execute it now..."  Many instruct-tuned models treat their own
assistant message as a completed response, and a trailing system
instruction is easy to ignore.

Changing the follow-up to role="user" makes the model see:
  assistant: plan
  user: "Execute this plan..."
which obligates the model to produce a new assistant response —
the tool-calling execution phase.

The follow-up message is appended only to session.context (LLM
context) and never to session.messages, so it is invisible in the
chat UI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>