fix(e2e): force foreground subagents in claude-code prompts

New Claude Code releases can run Task subagents in the background: the tool call returns immediately, the foreground turn ends, and the subagent's file changes and commit land after turn-end with no active session — so no checkpoint trailer or condensation happens and the checkpoint-advance assertions time out (CI failures on Linux and Windows).

Instruct the agent to run the subagent in the foreground and wait for it, keeping these tests on the synchronous path they are meant to cover. Tracking background-subagent work in the CLI itself (e.g. via the SubagentStop hook) is a separate product gap.