fix(ci): fix YAML parse error in deploy workflow caused by inner heredoc

The compose file was written via a bash << 'COMPOSE' heredoc nested inside
the YAML run: | block scalar. Lines like "name: petloft-staging" at column 0
cause the YAML parser to break out of the block scalar early, making the
entire workflow file invalid YAML — Gitea silently drops invalid workflows,
so no jobs triggered at all.

Fix: move compose.yml to deploy/staging/compose.yml in the repo, substitute
${REGISTRY} on the runner, base64-encode the result, and decode it on the VPS
inside the SSH session. No inner heredoc needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy/staging/compose.yml

@@ -0,0 +1,20 @@
+name: petloft-staging
+
+services:
+  storefront:
+    image: ${REGISTRY}/storefront:staging
+    restart: unless-stopped
+    ports:
+      - "3000:3000"
+    env_file:
+      - path: .env
+        required: false
+
+  admin:
+    image: ${REGISTRY}/admin:staging
+    restart: unless-stopped
+    ports:
+      - "3001:3001"
+    env_file:
+      - path: .env
+        required: false