Add user/channel allowlist for access control

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Summary

Add environment-based allowlists to control who can trigger the bot and in which channels.

Implementation

• ALLOWED_USERS - Comma-separated Discord user IDs
• ALLOWED_ROLES - Comma-separated Discord role IDs
• ALLOWED_CHANNELS - Comma-separated channel IDs

Check these before queueing any job in bot.ts. If lists are empty, allow all (current behavior).

Priority

This is the highest-priority safety feature - prevents random server members from spawning Claude sessions.

Labels

enhancement, security