fix(config): plan agent inherits model settings from prometheus when not explicitly configured

Previously, demoted plan agent only received { mode: 'subagent' } with no
model settings, causing fallback to step-3.5-flash. Now inherits all
model-related settings (model, variant, temperature, top_p, maxTokens,
thinking, reasoningEffort, textVerbosity, providerOptions) from the
resolved prometheus config. User overrides via agents.plan.* take priority.

Prompt, permission, description, and color are intentionally NOT inherited.

src/plugin-handlers/plan-model-inheritance.ts

@@ -0,0 +1,27 @@
+const MODEL_SETTINGS_KEYS = [
+  "model",
+  "variant",
+  "temperature",
+  "top_p",
+  "maxTokens",
+  "thinking",
+  "reasoningEffort",
+  "textVerbosity",
+  "providerOptions",
+] as const
+
+export function buildPlanDemoteConfig(
+  prometheusConfig: Record<string, unknown> | undefined,
+  planOverride: Record<string, unknown> | undefined,
+): Record<string, unknown> {
+  const modelSettings: Record<string, unknown> = {}
+
+  for (const key of MODEL_SETTINGS_KEYS) {
+    const value = planOverride?.[key] ?? prometheusConfig?.[key]
+    if (value !== undefined) {
+      modelSettings[key] = value
+    }
+  }
+
+  return { mode: "subagent" as const, ...modelSettings }
+}