chore(discord-presence): document the hardcoded Application ID

Replace the placeholder TODO now that the real public "PostHog Code"
Discord Application ID is set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: gantoine

apps/code/src/main/services/discord-presence/constants.ts

@@ -8,7 +8,7 @@
  * client. Register an application at https://discord.com/developers, upload the
  * art assets, then drop its ID here.
  */
-// TODO: replace with the real PostHog Code Discord Application ID.
+/** Public Discord Application ID for the "PostHog Code" Rich Presence app. */
 const DISCORD_CLIENT_ID = "1511709200017920020";
 
 export function getDiscordClientId(): string {