dsh-safemode-profile: Start dsh --profile safemode with zero third-party plugins

# Preface DSH's philosophy is "everything is a plugin," and profiles are used to segregate different plugin environments. The 'safemode' profile is positioned as a "clean environment": it is the most suitable choice for troubleshooting, comparison, or when you only need core capabilities. However, a safemode profile is essentially just a set of ordinary files under `$DSH_HOME/profiles/safemode/`. A single `dsh plugin --profile safemode add ...` command, a manual edit of `package.json`, or even a simple mistake in a script can compromise the cleanliness of this environment, and you may not necessarily discover it immediately. Post-hoc verification by checking files one by one and manual cleanup are both time-consuming and unreliable. dsh