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.

packages/utils/src/constants/index.ts

@@ -2,5 +2,6 @@ export * from './roles';
 export * from './org';
 export * from './content';
 export * from './domains';
+export * from './embeds';
 export * from './error-codes';
 export * from './agent';