Print hint when config load exceeds 500ms

## Summary
When `vp` spends more than ~500ms loading the local config we should close the run with a gentle hint so folks know where to look for tuning guidance.

## Motivation
Slow config discovery/merging usually means project-specific overrides or expensive plugin setup. Surfacing a pointer to the config docs right after the command finishes gives people a next step instead of silently eating the latency.

## Proposal
- Measure config load duration for every `vp` command that loads the project config.
- If it exceeds 500ms, append a one-line hint at the very end of the command output (after existing summaries) such as:
```
hint: config took 640ms to load – see https://viteplus.dev/config/ for cleanup tips
```
- Ensure:
- Only prints once per run
- Respects `--json` / `--silent` modes (do not emit in structured output scenarios)
- Uses the actual measured duration in the message

## Acceptance Criteria
- Commands that trigger slow config loads (>500ms) show the hint exactly once at the end.
- Fast runs (<500ms) show no extra output.
- Link points to https://viteplus.dev/config/.
- Covered by a test (unit or integration) that exercises both the <500ms and >500ms paths.