subagents: a worker persona, a live prompt fallback, and guards that scale ...

- assistant and designer_3d get their own subagent_system_prompt.txt; without
  one a sub-agent ran on the orchestrator's prompt, or since ce55325 on nothing
  but the scope anchor, and rarely finished the step it was given
- build the sub-agent system prompt in one place; the documented fallback to
  profile.system_prompt had become dead code
- the thinking-stall guard killed healthy cloud reasoners at 12k chars after
  13-35s; raise the char bound to 48k, keep the 60s wall, and make both settings
- a 300s run cap cut long ops work in half; raise it to 600s, configurable