fix: remove gray-matter dependency, use inline frontmatter parser

gray-matter was excluded from VSIX via .vscodeignore, causing runtime
   module not found error. Replaced with a zero-dependency inline parser.

   Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -15,7 +15,7 @@
   "categories": ["AI", "Other"],
   "keywords": ["copilot", "superpowers", "skills", "tdd", "brainstorming", "debugging"],
   "icon": "images/icon.png",
-  "activationEvents": [],
+  "activationEvents": ["onStartupFinished"],
   "main": "./out/extension.js",
   "contributes": {
     "chatParticipants": [
@@ -65,9 +65,6 @@
     "package": "vsce package",
     "publish": "vsce publish"
   },
-  "dependencies": {
-    "gray-matter": "^4.0.3"
-  },
   "devDependencies": {
     "@types/node": "^20.0.0",
     "@types/vscode": "^1.90.0",