docs(fleettools): booth — ephemeral is the default, KEEP is the exception (>24h persistence only)

docs/fleettools/booth.md

@@ -6,13 +6,20 @@
 Wiped **24h after last activity** unless kept. No auth (LAN/WG-internal), so never
 drop secrets in a booth.
 
+**Ephemeral is the default; KEEP is the exception, not the rule.** A booth is
+meant to be wiped by the 24h sweep. Only `booth keep` a booth whose data must
+**explicitly persist beyond 24h** — a standing board, or a record the operator
+will deliberately revisit — never "just in case". When in doubt, leave it
+ephemeral: a wiped render is cheap to regenerate, whereas a pile of stale kept
+booths is clutter that never cleans itself up.
+
 ## Show the operator something
 
 ```sh
 booth new <name>; booth add <name> <file>...     # or just cp/rsync into ~/booth-data/<name>/
 rsync -a ./out/ nh3-dev:booth-data/<name>/       # from another host
 booth url <name>                                 # -> http://10.100.10.50:8090/b/<name>/
-booth keep <name> / booth unkeep <name>          # exempt from / restore the 24h sweep
+booth keep <name> / booth unkeep <name>          # exempt from / restore the 24h sweep (the exception — only for data that must persist >24h)
 ```
 
 Auto-renders a gallery of images, webm+mp4 video and audio. A `<file>.txt` sidecar