feat: add Paperclip env example file with placeholder secrets

Add env/.env.example.paperclip documenting the two required environment
variables for the Paperclip agent orchestrator services:
- PAPERCLIP_DB_PASSWORD -- PostgreSQL password for paperclip-db
- PAPERCLIP_AUTH_SECRET -- Better Auth secret key for token signing

Users copy this to .env and fill in the secrets before deploying.

env/.env.example.paperclip

@@ -0,0 +1,26 @@
+# Paperclip Environment Variables
+# Copy this file to your .env (at the compose root or docker-compose working directory)
+# and fill in the secrets.
+#
+#   cp env/.env.example.paperclip .env
+#
+# Then reference it from compose.yml:
+#   env_file:
+#     - path: .env
+#       required: true
+
+# ---------------------------------------------------------------------------
+# Database
+# ---------------------------------------------------------------------------
+# PostgreSQL password for the paperclip-db service.
+# Generate a strong random password:
+#   openssl rand -base64 32
+PAPERCLIP_DB_PASSWORD=change_me_to_a_strong_random_password
+
+# ---------------------------------------------------------------------------
+# Authentication
+# ---------------------------------------------------------------------------
+# Secret key used by Better Auth for signing and verifying tokens.
+# Generate a strong random secret:
+#   openssl rand -base64 32
+PAPERCLIP_AUTH_SECRET=change_me_to_a_strong_random_secret