Add IS_WSL2 export to runtime.sh and replace all inline
grep-based WSL2 guards in administration scripts with a
source + variable check for consistency.
Affected scripts:
- scripts/administration/systemd/enable/wsl2.sh
- scripts/administration/tls/trust/wsl2.sh
- scripts/administration/network/dns/setup/wsl.sh
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>