fix: remove generatePrompt method to match Mode interface

Remove the generatePrompt method from agent mode since it's not part of the Mode interface in the current main branch. Also remove unused PreparedContext import.

src/modes/agent/index.ts

@@ -1,7 +1,6 @@
 import * as core from "@actions/core";
 import type { Mode, ModeOptions, ModeResult } from "../types";
 import { isAutomationContext } from "../../github/context";
-import type { PreparedContext } from "../../create-prompt/types";
 import { prepareMcpConfig } from "../../mcp/install-mcp-server";
 
 /**
@@ -94,18 +93,4 @@ export const agentMode: Mode = {
       mcpConfig: mcpConfig,
     };
   },
-
-  generatePrompt(context: PreparedContext): string {
-    // Agent mode uses override or direct prompt, no GitHub data needed
-    if (context.overridePrompt) {
-      return context.overridePrompt;
-    }
-
-    if (context.directPrompt) {
-      return context.directPrompt;
-    }
-
-    // Minimal fallback - repository is a string in PreparedContext
-    return `Repository: ${context.repository}`;
-  },
 };