refactor(config-handler): separate plan prompt into dedicated configuration (#1413)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/plugin-handlers/config-handler.test.ts

@@ -176,10 +176,12 @@ describe("Plan agent demote behavior", () => {
     await handler(config)
 
     // then
-    const agents = config.agent as Record<string, { mode?: string; name?: string }>
+    const agents = config.agent as Record<string, { mode?: string; name?: string; prompt?: string }>
     expect(agents.plan).toBeDefined()
     expect(agents.plan.mode).toBe("subagent")
     expect(agents.plan.name).toBe("plan")
+    expect(agents.plan.prompt).toBe("original plan prompt")
+    expect(agents.plan.prompt).not.toBe(agents.prometheus?.prompt)
   })
 
   test("prometheus should have mode 'all' to be callable via delegate_task", async () => {

src/plugin-handlers/config-handler.ts

@@ -210,6 +210,12 @@ 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";
 
@@ -241,11 +247,6 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
       }
 
       if (plannerEnabled) {
-        const { name: _planName, mode: _planMode, ...planConfigWithoutName } =
-          configAgent?.plan ?? {};
-        const migratedPlanConfig = migrateAgentConfig(
-          planConfigWithoutName as Record<string, unknown>
-        );
         const prometheusOverride =
           pluginConfig.agents?.["prometheus"] as
             | (Record<string, unknown> & {
@@ -365,7 +366,8 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
         ? {
             ...agentConfig["prometheus"],
             name: "plan",
-            mode: "subagent" as const 
+            mode: "subagent" as const,
+            ...(planPrompt ? { prompt: planPrompt } : {}),
           }
         : undefined;