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

[zerone0x]

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

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[fengmk2]

The DX issue reported should automatically detect existing Agent types, but should not directly skip the agent's content updates
https://github.com/voidzero-dev/vite-plus/blob/main/packages/cli/src/utils/agent.ts#L501

[zerone0x]

The DX issue reported should automatically detect existing Agent types, but should not directly skip the agent's content updates main/packages/cli/src/utils/agent.ts#L501

Hey! Thanks for the feedback! the early one was too aggressive.

Updated approach: auto-detection is now moved to the call site in migration/bin.ts, aligning with the existing pattern in create/bin.ts. selectAgentTargetPaths remains unchanged — the detection only skips the selector prompt, while conflict resolution and content updates (append/skip) still proceed normally.