Add Discord slash command integration

[pntech20]

Summary

• adds a minimal /sanctifier Discord bot with explain, latest, and status subcommands
• includes command registration, bot runtime, finding-code explanations, latest-report formatting, and endpoint status checks
• documents setup, environment variables, and deployment notes for staging or production servers

Checks

• npm test
• node --check src/commands.js; node --check src/bot.js; node --check src/register-commands.js
• git diff --check

Closes #850

[vercel]

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