Add startup performance controls

[sylvesterdamgaard]

Summary

• add --skip-permission-setup / CBOX_INIT_SKIP_PERMISSION_SETUP=true for pre-baked images
• add --skip-runtime-validation / CBOX_INIT_SKIP_RUNTIME_VALIDATION=true to avoid php-fpm/nginx validation on every boot
• add CBOX_INIT_STARTUP_TIMING=true to log startup phase durations
• update dry-run output and environment variable docs
• add unit coverage for the new env/timing helpers

Verification

• git diff --check
• go test ./cmd/cbox-init could not be run locally: go is not installed in this environment