Add Burn 451 to Proprietary

[Fisher521]

Adding Burn 451 — an AI-native read-later + knowledge vault for note-takers and knowledge workers.

Brief: Save any article with a 24-hour timer that forces read-or-delete. Kept articles become a permanent Vault that AI agents (Claude, Cursor, Claude Code) can query in real time via a built-in MCP server. iOS app + Web + Chrome extension.

Checklist (per contributing.md):

• Primarily for note-taking or knowledge management — it's a PKM-adjacent read-later and vault
• Entry concise, 1-2 sentences, ends with a period
• Link goes to the official site
• Icons: 📕 (proprietary/database format — uses Supabase, not raw markdown files), 🍎 (iOS app confirmed), 🔁 (cloud sync via Supabase). No 🤖 (no Android yet), no 🔒 (E2EE not documented).
• Placed in alphabetical order inside ## Proprietary — between Bear and Capacities
• Not a duplicate — searched README for "burn" and "451"
• Actively maintained — iOS app + web app live, product updates shipped within the past week