feat: upgrade migration system — versioned fix scripts for broken state

Adds gstack-upgrade/migrations/ directory with version-keyed bash scripts
that run automatically during /gstack-upgrade (Step 4.75, after ./setup).
Each script is idempotent and handles state fixes that setup alone can't
cover. First migration: v0.15.2.0.sh runs gstack-relink to fix stale
directory symlinks from pre-v0.15.2.0 installs.

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

gstack-upgrade/migrations/v0.15.2.0.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Migration: v0.15.2.0 — Fix skill directory structure for unprefixed discovery
+#
+# What changed: setup now creates real directories with SKILL.md symlinks
+# inside instead of directory symlinks. The old pattern (qa -> gstack/qa)
+# caused Claude Code to auto-prefix skills as "gstack-qa" even with
+# --no-prefix, because Claude sees the symlink target's parent dir name.
+#
+# What this does: runs gstack-relink to recreate all skill entries using
+# the new real-directory pattern. Idempotent — safe to run multiple times.
+#
+# Affected: users who installed gstack before v0.15.2.0 with --no-prefix
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")/../.." && pwd)"
+
+if [ -x "$SCRIPT_DIR/bin/gstack-relink" ]; then
+  echo "  [v0.15.2.0] Fixing skill directory structure..."
+  "$SCRIPT_DIR/bin/gstack-relink" 2>/dev/null || true
+fi