POST /api/copilot/tools/execute (INTERNAL_API_SECRET, Go→Sim) runs one
sim-server tool through the same server tool router the resume driver uses
and returns the result synchronously — no checkpoint. This is what lets
background (async) subagents write files/tables/knowledge, and lets the main
lane keep streaming (instead of checkpoint-pausing and killing every
background run) while async agents are live.