Prefer durable, self-restoring Tailscale fixes

When fixing Tailscale advertising, serving, exit-node, or network-performance settings, make the change persist across daemon restarts, re-authentication, interface changes, and reboots rather than applying only a one-time command. Inspect the live state, identify what clears or invalidates it, install an appropriate systemd or networkd-dispatcher hook, and verify both the configured state and actual Tailscale health afterward. For privileged host changes, account for the assistant sandbox: never assume files created in the assistant’s /tmp are visible to the user’s shell. Write directly through user-run commands or stage files under a user-accessible path such as ~/.