Add Discord bot support with shared conversation state

[owenqwenstarsky]

Summary

• add a Discord bot runtime that starts with the buddy server and supports DMs, mentions, slash commands, approval prompts, and tool activity summaries
• share conversation storage between the CLI and Discord while persisting active session pointers in current.json
• extend config and prompt handling for Discord-specific behavior, including allowed usernames and Discord-friendly response formatting

Testing

• npm run check