chore: bootstrap gstack team mode

Per gstack team-mode install: adds a PreToolUse hook that blocks
skill usage when gstack isn't installed globally, so contributors
are prompted to install it. Un-ignores the two required files
(.claude/settings.json, .claude/hooks/check-gstack.sh) while
keeping settings.local.json and other Claude state ignored.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.claude/settings.json

@@ -0,0 +1,15 @@
+{
+  "hooks": {
+    "PreToolUse": [
+      {
+        "matcher": "Skill",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "\"$CLAUDE_PROJECT_DIR/.claude/hooks/check-gstack.sh\""
+          }
+        ]
+      }
+    ]
+  }
+}