[codex] Fix Cmd+Enter submit when idle

[SawyerHood]

Summary

• Wire the follow-up prompt modifier shortcut only when steer submit is actually available.
• Let Cmd+Enter fall through to the normal prompt submit path on idle/non-working threads.
• Add focused coverage for active steer behavior and idle normal-submit behavior with queued/current drafts.

Validation

• PATH="/Users/sawyerhood/Library/Caches/pnpm/dlx/mgdbhd5zwcpibuaobycjdzyaga/19e28e4c4fd-bb5d/node_modules/.pnpm/node@22.12.0/node_modules/node/bin:$PATH" pnpm exec turbo run test --filter=@bb/app -- src/components/promptbox/FollowUpPromptBox.test.tsx src/views/thread-detail/ThreadDetailPromptArea.test.tsx
• PATH="/Users/sawyerhood/Library/Caches/pnpm/dlx/mgdbhd5zwcpibuaobycjdzyaga/19e28e4c4fd-bb5d/node_modules/.pnpm/node@22.12.0/node_modules/node/bin:$PATH" pnpm exec turbo run typecheck --filter=@bb/app

Note: the ambient shell is on Node 25.3.0, which breaks jsdom localStorage before tests load. Validation was run with Node 22.12.0 from .nvmrc first in PATH.