refactor(core): own todo event schema

[kitlangton]

Summary

• make Core the single owner of the shared Todo schema and todo.updated event definition
• reuse those definitions from the existing OpenCode todo service
• preserve the current event payload, route behavior, and generated API output

Why

Core and OpenCode independently registered equivalent todo.updated definitions in the global EventV2 registry. Reusing the Core definition removes import-order ownership and future schema-drift risk without migrating either service.

Verification

• bun run test test/session-todo.test.ts
• Core typecheck
• OpenCode typecheck
• full generation produced no generated artifact changes
• pre-push workspace typecheck (23/23 tasks)