feat(discord): support GOCLAW_DISCORD_TOKEN env var for channel config (#6)

Add env var overlay and auto-enable for Discord channel, matching the
existing Telegram pattern. Update .env.example and docker-compose.yml.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -17,6 +17,7 @@ GOCLAW_ENCRYPTION_KEY=
 
 # --- Channels (optional) ---
 GOCLAW_TELEGRAM_TOKEN=
+GOCLAW_DISCORD_TOKEN=
 
 # --- Database (only for non-Docker deployments) ---
 # Docker Compose auto-builds this from POSTGRES_USER/PASSWORD/DB.