feat: add bin/gstack-slug helper + migrate all inline SLUG computation

Extract the opaque SLUG sed pipeline into a shared 5-line shell script.
Replace 8 inline copies across templates with eval $(gstack-slug).
Sanitizes branch names (/ → -) to prevent subdirectory creation.

bin/gstack-slug

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# gstack-slug — output project slug and sanitized branch name
+# Usage: eval $(gstack-slug)  → sets SLUG and BRANCH variables
+# Or:    gstack-slug           → prints SLUG=... and BRANCH=... lines
+set -euo pipefail
+SLUG=$(git remote get-url origin 2>/dev/null | sed 's|.*[:/]\([^/]*/[^/]*\)\.git$|\1|;s|.*[:/]\([^/]*/[^/]*\)$|\1|' | tr '/' '-')
+BRANCH=$(git rev-parse --abbrev-ref HEAD 2>/dev/null | tr '/' '-')
+echo "SLUG=$SLUG"
+echo "BRANCH=$BRANCH"