refactor(config): make Prometheus model independent from plan agent config

- Prometheus no longer inherits model from plan agent configuration
- Fallback chain: session default model -> claude-opus-4-5
- Removes coupling between Prometheus and legacy plan agent settings

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/plugin-handlers/config-handler.ts

@@ -176,7 +176,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
           pluginConfig.agents?.["Prometheus (Planner)"];
         const defaultModel = config.model as string | undefined;
         const prometheusBase = {
-          model: (migratedPlanConfig as Record<string, unknown>).model ?? defaultModel,
+          model: defaultModel ?? "anthropic/claude-opus-4-5",
           mode: "primary" as const,
           prompt: PROMETHEUS_SYSTEM_PROMPT,
           permission: PROMETHEUS_PERMISSION,