Add discord-agent files and enable reply notifications

- Add discord_agent.py with reply() instead of send() for user notifications
- Add Discord bot Dockerfile and requirements.txt
- Add bot cogs (base_cog.py and integration_cog.py)
- Update .gitignore to track discord-agent directory
- Bot now replies to messages triggering notifications for users

.gitignore

@@ -8,6 +8,8 @@
 !README.md
 !RESTORE.md
 !AGENTS.md
+!discord-agent/
+!discord-agent/**
 
 # Never track actual secrets
 .env