fix: simplify codex Slack channel settings

[LIU9293]

Summary

• let Codex run without a configured channel model so Ode skips the --model flag and stops warning users to configure one
• keep Slack channel settings aligned by treating Codex model selection as optional while making working directory required in the modal
• bump the app version to 0.1.20 and cover the no-model Codex CLI case with a test

Testing

• bun test packages/agents/test/cli-command.test.ts
• bun run typecheck (currently fails due to existing packages/web-ui missing module/type issues)