config: subagents -> tinfoil/deepseek-v4-flash (cheaper, pay-per-use)

Switch the general subagent model from opencode/deepseek-v4-pro (zen
endpoint, subject to workspace balance limits) to tinfoil/deepseek-v4-flash
(pay-per-use API, no rolling balance limit, cheaper than pro).

This completes the fleet-wide move to deepseek-flash on tinfoil: the upgrader
parent (LOOP_MODEL) and report (REPORT_MODEL) are set in upgrader.env, and
this changes the subagents. The hourly supervisor stays on glm-5.2.

Verified: tinfoil endpoint accepts deepseek-v4-flash (HTTP 200, 2.3s response).
Trial run next Friday (2026-08-21) to evaluate flash capability on the
upgrade workload; fall back to pro if it struggles on complex recipes.

opencode.json

@@ -2,7 +2,7 @@
   "$schema": "https://opencode.ai/config.json",
   "agent": {
     "general": {
-      "model": "opencode/deepseek-v4-pro"
+      "model": "tinfoil/deepseek-v4-flash"
     }
   }
 }