Adds a root-user guard to Taskfile's _preflight task (the shared
precondition for all local task commands) and to five scripts that can
be invoked directly outside of `task`. Also adds `rm -f` before
writing ~/.ssh/dagger_key in setup_dagger_remote.sh to safely remove
any existing file or symlink before writing the new key.
Closes#414
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>