feat(embeds): add embed.classroomio.com public widget host

Serve widgets from short public URLs via tenant-router with R2-backed
assets and runtime API_URL injection, so copy-paste snippets hide infra
paths. Update widget payloads, CSP, upload purge URLs, and dev-only API
inject; document format/stage workflow in AGENTS.md and fix ui-prefix-check
YAML syntax.

apps/tenant-router/tsconfig.json

@@ -12,5 +12,5 @@
     "isolatedModules": true,
     "resolveJsonModule": true
   },
-  "include": ["src/**/*.ts"]
+  "include": ["src/**/*.ts", "../../packages/utils/src/constants/embeds.ts"]
 }