Enforce single-instance in socktop-gestures instead of documenting it

The documented restart was `pkill -x lisgd && socktop-gestures &`. pkill exits
non-zero when it matches nothing, so `&&` short-circuits and the daemon
silently does not start when none was running -- you swipe, get nothing, and
reasonably blame the config change you just made. socktop-gestures exec'd lisgd
with no guard, so nothing downstream caught it.

Rather than only fixing the one-liner, move the invariant into the tool: it now
exits non-zero if a daemon is already running, and --replace stops the old one
first (waiting for it to actually exit). This is the failure the README itself
flags as gotcha 3, so a false success there is expensive.

Verified against a live daemon: refusal, --help, bad-option exit codes, and a
clean pid handover under --replace with the carousel still working.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -80,10 +80,15 @@ Everything lives in **`/usr/local/etc/socktop-swipe.env`**. Edit it there, not i
 the repo copy. After changing it, restart the daemon and rebuild the session:
 
 ```sh
-pkill -x lisgd && socktop-gestures &
+socktop-gestures --replace &   # restarts the daemon, running or not
 socktop-rack
 ```
 
+Use `--replace` rather than `pkill -x lisgd && socktop-gestures`: `pkill` exits
+non-zero when it matches nothing, so that one-liner silently starts **nothing**
+if no daemon was running — you would swipe, get no response, and reasonably
+blame the config change.
+
 | Option | Default | What it does |
 | --- | --- | --- |
 | `SOCKTOP_HOSTS` | four Pi profiles | socktop profile names, **in swipe order**. Any count works; layout and carousel adapt. |
@@ -113,6 +118,8 @@ systemctl --user enable --now socktop-swipe
 
 Either way, **run exactly one gesture daemon**. lisgd does not grab the input
 device exclusively, so two instances make every swipe fire twice.
+`socktop-gestures` enforces this: it exits non-zero if one is already running,
+unless given `--replace`.
 
 ## Uninstall
 
@@ -160,6 +167,10 @@ Also ensure tmux mouse mode is **off**; `socktop-rack` sets this.
 
 Two lisgd instances are running. `pgrep -x lisgd` should show exactly one.
 
+`socktop-gestures` refuses to start if a daemon is already running, so this
+should not happen via the normal path; `--replace` restarts cleanly. It can
+still occur if lisgd was launched by hand.
+
 ### 4. Swipes do nothing at all
 
 In order: is the session running (`tmux ls`)? Is the daemon running