feat(maestro): make installer re-run safe (auto-detect = update)

The wizard now treats existing state as "we're updating, not installing":

- If `~/.config/maestrod/config.toml` exists, skip the configure prompt with
  a clear notice. Sets `configured = true` so the systemd start logic still
  makes sense.
- If the systemd user unit already exists, refresh the file in place, skip
  the enable prompt (it's either already enabled or the user said no last
  time), and ask to `restart` instead of `start` — so the new binary
  actually takes effect.

Adds `SystemctlAction::Restart` and a test for it. Same `./install.sh` works
for both first install and subsequent updates; no `--update` flag.

Refs NAP-225.

docs/INSTALL.md

@@ -14,7 +14,12 @@ The repo is private, so grab `install.sh` while signed in to gitea, then run it:
 
 The bootstrap script builds `maestro-installer` into a temp directory, runs the
 wizard, and cleans up — only `maestrod` and `maestro` end up persisted in
-`~/.cargo/bin`. Re-run any time to update; `cargo install` overwrites in place.
+`~/.cargo/bin`.
+
+**Updating is the same command.** Re-running `./install.sh` detects an
+existing `~/.config/maestrod/config.toml` and existing systemd unit, skips the
+configure prompt, refreshes the unit, and runs `systemctl --user restart
+maestrod` instead of `start`. No `--update` flag needed.
 
 Inside the script, `cargo install --git https://git.naps.pt/naps62/maestro …`
 uses your already-configured git credentials for this host (HTTPS helper or