build: add remote-gpm-install (single out-of-plugins.txt package)

For installing remote-only plugins like git-sync that are deliberately
excluded from plugins.txt. Usage: make remote-gpm-install-prod PKG=git-sync

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Np4cMQLF77i664CAQXySzU

Makefile

@@ -27,7 +27,7 @@ REMOTE_TARGETS := remote-env-setup remote-env-remove remote-wipe remote-install
 	remote-fetch remote-fetch-content remote-install-plugins remote-update-plugins \
 	remote-upgrade-grav remote-git-sync-disable remote-git-sync-enable \
 	remote-content-status remote-clean remote-diag remote-apply-env \
-	remote-maintenance-on remote-maintenance-off
+	remote-gpm-install remote-maintenance-on remote-maintenance-off
 ENVS := test prod
 
 guard-env:
@@ -178,6 +178,13 @@ remote-content-status: guard-env
 remote-clean: guard-env
 	$(SSH) "cd $(WEBROOT) && php bin/grav clearcache"
 
+# Install a single GPM package on the server (e.g. git-sync, which is
+# intentionally NOT in plugins.txt — it is remote-only).
+# Usage: make remote-gpm-install-prod PKG=git-sync
+remote-gpm-install: guard-env
+	@test -n "$(PKG)" || { echo "ERROR: set PKG=<plugin-slug>"; exit 1; }
+	$(SSH) "cd $(WEBROOT) && php bin/gpm index -f && php bin/gpm install $(PKG) -y && php bin/grav clearcache"
+
 # Deploy per-environment Grav config overrides to the server's
 # user/env/<WEB_HOST>/config/ tree (deep-merged over the committed config).
 # Source of truth: deploy/env/$(ENV)/system.yaml (version-controlled). This