Log SteamVR-started instances to logs/<exe>.log

The manifest passes --log-file, so a settings error (CFG-2) ends up in a
log file in the install folder. Package README and docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/STEAM-FRAME.md

@@ -42,6 +42,9 @@ scripts/deploy-to-frame.sh steamos@<frame> # rsync the newest package to ~/<exe>
 
 A release folder contains the program, `install.sh` (install or update: stops a running
 instance, registers with SteamVR, starts it), `uninstall.sh`, `README.txt` and the licenses.
+The app logs to `logs/frame-assistant.log` in that folder (passed by the manifest), and
+needs a settings file first (`~/.config/frameassistant/config.json`, from
+`share/frameassistant/config.example.json`; see README.md, "Settings").
 SteamVR stores the folder's absolute path, so after moving the folder, run `install.sh`
 again.