Add config.json.example with placeholder credentials.

Document the expected config shape for first-time setup without committing secrets.

Co-authored-by: Cursor <cursoragent@cursor.com>

config.json.example

@@ -0,0 +1,11 @@
+{
+  "host": "mattermost.example.com",
+  "login_mode": "gitlab",
+  "username": "your.username",
+  "gitlab_url": "https://gitlab.example.com",
+  "token": null,
+  "cookie_file": "cookies.pkl",
+  "download_files": true,
+  "throttle": true,
+  "retry": true
+}