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/config-handler.ts

@@ -32,6 +32,7 @@ import { AGENT_NAME_MAP } from "../shared/migration";
 import { AGENT_MODEL_REQUIREMENTS } from "../shared/model-requirements";
 import { PROMETHEUS_SYSTEM_PROMPT, PROMETHEUS_PERMISSION } from "../agents/prometheus";
 import { DEFAULT_CATEGORIES } from "../tools/delegate-task/constants";
+import { buildPlanDemoteConfig } from "./plan-model-inheritance";
 import type { ModelCacheState } from "../plugin-state";
 import type { CategoryConfig } from "../config/schema";
 
@@ -385,8 +386,10 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
         : {};
 
       const planDemoteConfig = shouldDemotePlan
-           ? { mode: "subagent" as const
-          }
+        ? buildPlanDemoteConfig(
+            agentConfig["prometheus"] as Record<string, unknown> | undefined,
+            pluginConfig.agents?.plan as Record<string, unknown> | undefined,
+          )
         : undefined;
 
       config.agent = {