fix(config): prevent plan agent from inheriting prometheus prompt on demote

Plan agent demote now only sets mode to 'subagent' without spreading
prometheus config. This ensures plan agent uses OpenCode's default
prompt instead of inheriting prometheus prompt.

src/plugin-handlers/config-handler.ts

@@ -195,7 +195,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
     const plannerEnabled =
       pluginConfig.sisyphus_agent?.planner_enabled ?? true;
     const replacePlan = pluginConfig.sisyphus_agent?.replace_plan ?? true;
-    const shouldReplacePlan = plannerEnabled && replacePlan;
+    const shouldDemotePlan = plannerEnabled && replacePlan;
 
     type AgentConfig = Record<
       string,
@@ -211,12 +211,6 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
     };
     const configAgent = config.agent as AgentConfig | undefined;
 
-    const { name: _planName, mode: _planMode, ...planConfigWithoutName } =
-      configAgent?.plan ?? {};
-    const planPrompt = (migrateAgentConfig(
-      planConfigWithoutName as Record<string, unknown>
-    ) as { prompt?: string }).prompt;
-
     if (isSisyphusEnabled && builtinAgents.sisyphus) {
       (config as { default_agent?: string }).default_agent = "sisyphus";
 
@@ -345,7 +339,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
           Object.entries(configAgent)
             .filter(([key]) => {
               if (key === "build") return false;
-              if (key === "plan" && shouldReplacePlan) return false;
+              if (key === "plan" && shouldDemotePlan) return false;
               // Filter out agents that oh-my-opencode provides to prevent
               // OpenCode defaults from overwriting user config in oh-my-opencode.json
               // See: https://github.com/code-yeongyu/oh-my-opencode/issues/472
@@ -363,13 +357,8 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
         ? migrateAgentConfig(configAgent.build as Record<string, unknown>)
         : {};
 
-      const planDemoteConfig = shouldReplacePlan && agentConfig["prometheus"]
-        ? {
-            ...agentConfig["prometheus"],
-            name: "plan",
-            mode: "subagent" as const,
-            ...(typeof planPrompt === "string" ? { prompt: planPrompt } : {}),
-          }
+      const planDemoteConfig = shouldDemotePlan
+        ? { mode: "subagent" as const }
         : undefined;
 
       config.agent = {