fix(cli): skip agent selector when AGENTS.md already exists

When running `vp migrate`, detect existing agent instruction files
(AGENTS.md, CLAUDE.md, etc.) in the project root and reuse them
instead of showing the interactive agent selector prompt.

Fixes #903

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zerone0x

packages/cli/src/migration/bin.ts

@@ -343,6 +343,7 @@ async function collectMigrationPlan(
   const selectedAgentTargetPaths = await selectAgentTargetPaths({
     interactive: options.interactive,
     agent: options.agent,
+    projectRoot: rootDir,
     onCancel: () => cancelAndExit(),
   });
 

packages/cli/src/utils/agent.ts

@@ -196,17 +196,27 @@ const AGENT_INSTRUCTIONS_END_MARKER = '<!--VITE PLUS END-->';
 export async function selectAgentTargetPaths({
   interactive,
   agent,
+  projectRoot,
   onCancel,
 }: {
   interactive: boolean;
   agent?: AgentSelection;
+  projectRoot?: string;
   onCancel: () => void;
 }) {
   // Skip entirely if --no-agent is passed
   if (agent === false) {
     return undefined;
   }
 
+  // If agent files already exist in the project, reuse them instead of prompting
+  if (interactive && !agent && projectRoot) {
+    const existingPaths = detectExistingAgentTargetPaths(projectRoot);
+    if (existingPaths) {
+      return existingPaths;
+    }
+  }
+
   if (interactive && !agent) {
     const selectedAgents = await prompts.multiselect({
       message: 'Which agents are you using?',