fix(cli): skip agent selector when AGENTS.md already exists (#937)

## Summary
- When running `vp migrate`, the agent selector prompt is now skipped if
agent instruction files (e.g. `AGENTS.md`, `CLAUDE.md`) already exist in
the project root
- Uses the existing `detectExistingAgentTargetPaths()` utility to check
for files before showing the interactive multiselect
- Adds an optional `projectRoot` parameter to `selectAgentTargetPaths()`
to enable detection without breaking existing callers

Fixes #903

## Test plan
- [ ] Run `vp migrate` in a project with an existing `AGENTS.md` — agent
selector prompt should be skipped
- [ ] Run `vp migrate` in a project without any agent files — agent
selector prompt should appear as before
- [ ] Run `vp migrate --no-agent` — should still skip agent setup
entirely
- [ ] Run `vp migrate --agent claude` — should still use the explicit
agent selection

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: MK (fengmk2) <fengmk2@gmail.com>

Author: 3 people

packages/cli/src/migration/bin.ts

@@ -14,6 +14,7 @@ import {
 } from '../types/index.js';
 import {
   detectAgentConflicts,
+  detectExistingAgentTargetPaths,
   selectAgentTargetPaths,
   writeAgentInstructions,
 } from '../utils/agent.js';
@@ -339,12 +340,19 @@ async function collectMigrationPlan(
     }
   }
 
-  // 3. Agent selection
-  const selectedAgentTargetPaths = await selectAgentTargetPaths({
-    interactive: options.interactive,
-    agent: options.agent,
-    onCancel: () => cancelAndExit(),
-  });
+  // 3. Agent selection (auto-detect existing agent files to skip the selector prompt)
+  const existingAgentTargetPaths =
+    options.agent !== undefined || !options.interactive
+      ? undefined
+      : detectExistingAgentTargetPaths(rootDir);
+  const selectedAgentTargetPaths =
+    existingAgentTargetPaths !== undefined
+      ? existingAgentTargetPaths
+      : await selectAgentTargetPaths({
+          interactive: options.interactive,
+          agent: options.agent,
+          onCancel: () => cancelAndExit(),
+        });
 
   // 4. Agent conflict detection + prompting
   const agentConflicts = await detectAgentConflicts({