hook: spawn-gate.sh

Part 3 · EACH TOOL · Slide 47 of 60

Event: PreToolUse on Agent · 15 s.
Reads the spawn payload; exit 2 with a deny decision when the
model is omitted, when subagent_type is "fork", or when the
model is haiku. Sonnet and opus pass. The override variable
permits loudly (it states itself in the transcript).

   Agent call ──► gate ──► allow (0) / deny (2 + reason)