feat: Slack/Discord bot — `/extract <url>` returns tokens in-thread

[Manavarya09]

Summary

`/extract https://stripe.com\` in any channel returns:

• A grade badge image
• Top 8 colors as Slack/Discord-native chips
• Type scale + family
• Link to full grade.html

`/battle stripe.com vercel.com`, `/remix stripe.com cyberpunk` work the same way.

Why

Designers + PMs live in Slack/Discord. Bringing designlang inline removes friction. Every shared extract becomes free distribution.

Scope

~5h. Slack bolt + Discord.js wrappers around `/api/extract`. Mostly UI-formatting work.