chore(upgrade): migrate feature-discovery markers to GSTACK_HOME

Follow-through on the #2748 absorption: existing installs answered the
continuous-checkpoint and model-overlay prompts with markers beside the
install; v1.78 reads them from GSTACK_HOME. Copy them once so nobody gets
re-prompted. Idempotent, non-fatal.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

gstack-upgrade/migrations/v1.78.0.0.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Migration: v1.78.0.0 — carry feature-discovery acknowledgements to GSTACK_HOME
+# (#2728 absorption follow-through).
+#
+# Why a migration: pre-v1.78, `gstack-skill-start` gated the one-time feature
+# prompts (.feature-prompted-continuous-checkpoint, .feature-prompted-model-
+# overlay) on marker files BESIDE THE INSTALL — which, for project-local
+# symlink installs, resolved into the project repo and left machine-state
+# droppings in checkouts (#2728). v1.78 reads them from GSTACK_HOME. Without
+# this copy, every existing install that already answered those prompts gets
+# re-prompted once per feature.
+#
+# Idempotent: existing destination markers are left untouched. Non-fatal
+# throughout — a failed copy just means one benign re-prompt.
+set -u
+
+INSTALL_DIR="${GSTACK_INSTALL_DIR:-$HOME/.claude/skills/gstack}"
+GH="${GSTACK_HOME:-$HOME/.gstack}"
+
+mkdir -p "$GH" 2>/dev/null || exit 0
+
+for m in .feature-prompted-continuous-checkpoint .feature-prompted-model-overlay; do
+  if [ -f "$INSTALL_DIR/$m" ] && [ ! -f "$GH/$m" ]; then
+    touch "$GH/$m" 2>/dev/null && echo "migrated: $m → $GH"
+  fi
+done
+
+exit 0