When a subagent stops (timeout, max iterations, thinking stall, user stop),
it now returns a structured progress report built from its local message
context, so the parent agent knows what tools were called and what was
accomplished before the stop.

- Add _build_progress_report() to SubAgentRunner
- Report includes: turn number, assistant text, tool calls with results
- Prepended to result_text for every stop reason (completed also gets it)
- Updated test_run_ephemeral_complete to expect the report prefix

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