docs: add .env.example with OAuth placeholders (secrets not in repo)

Author: jaredzhu3

.env.example

@@ -0,0 +1,12 @@
+## Copy to .env and fill in as needed.
+
+# Google OAuth (required for Google login flows)
+# Obtain from Google Cloud Console > Credentials > OAuth 2.0 Client IDs
+GOOGLE_OAUTH_CLIENT_ID=
+GOOGLE_OAUTH_CLIENT_SECRET=
+
+# Optional: Other providers used across the repo (leave blank unless needed)
+#OPENAI_API_KEY=
+#GEMINI_API_KEY=
+#TAVILY_API_KEY=
+