pkgbuild: Install auto-update launch shim

Replace the /usr/bin/grok-bot (and sand) plain symlink with a launch
shim that checks the latest Gitea release, pacman -U's a newer
.pkg.tar.zst when available, then execs the real binary under
/opt/Grok Bot/. Skip with GROK_BOT_NO_UPDATE=1; fail soft on
network/API errors. Bump pkgrel for the packaging change.

#2

README.md

@@ -57,5 +57,6 @@ Scheduled Gitea Actions will do the same and build a release package.
 
 ## Notes
 
-- Linux has no in-app updater for Grok Bot; re-run update + `makepkg -si` (or install the newer release).
+- `/usr/bin/grok-bot` (and `sand`) is a launch shim: unless `GROK_BOT_NO_UPDATE=1`, it checks the latest Gitea release and may `pacman -U` a newer `.pkg.tar.zst` before exec'ing `/opt/Grok Bot/…`. Network/API failures still launch the installed binary.
+- Upstream Linux builds still have no in-app updater for the Electron app itself; `scripts/update.sh` bumps the PKGBUILD from Cursor's feed when packaging a new upstream version.
 - `chrome-sandbox` is left non-setuid because the install path contains a space; user namespaces cover sandboxing on CachyOS.