Show detected package manager at startup (verbose mode)

Add an optional verbose output that shows which package manager was detected and why. This helps users confirm that spm is using the right tool, and is useful for debugging detection issues.

### Expected behavior

```
$ spm install --verbose
→ using yarn (detected via yarn.lock)
yarn install v4.1.0
...
```

This could also be enabled via an environment variable like `SPM_VERBOSE=1`.