SubagentsPi container validation #2 (first-action phrasing) — fix-git PASS with live delegation

Rerun of the single-task SubagentsPi container validation after strengthening the workflow append ("make your very FIRST action a subagent tool call with agent scout" — still positively phrased, no prohibitions). Compare fast__qwen3.6-35b-a3b__20260706-225730 (same task, advisory phrasing, zero delegation).

• fix-git PASS, 1m55s elapsed.
• 18 subagent tool calls in-container: scout exploration first, then planner (fed the task + scout findings), then reviewer after the fix — the staged workflow actually engaged, with the model doing the implement/test stages itself (the self-serve fallback by design).
• Child sessions inherited the global extension (guards active, workflow append suppressed via PI_SUBAGENT_CHILD).
• rm -rf .pi-subagents cleanup ran; no .pi-subagents droppings in the workdir artifacts.

This closes the "live subagent tool exercised only by a container run" caveat: install → global extension → child provider inheritance → delegation → cleanup all proven in the real Harbor pipeline. Wall-clock cost of delegation on a trivial task: ~2× the no-delegation run (1m55s vs 1m04s) — expect much larger multipliers on real tasks (per-switch re-prefills on the single llama-swap slot).