Add TUI option to preserve Vim mode after submitting prompts

Summary

Vim mode in the Codex CLI TUI currently returns to Normal mode after a prompt is submitted. For users who mostly continue typing follow-up prompts, it would be useful to preserve the current Vim mode after prompt submit, especially Insert mode.

Proposed behavior

Add a TUI config option, for example:

[tui]
vim_mode_after_submit = "preserve"

Suggested values:

• normal default: keep current behavior
• preserve: keep active Vim mode after successful prompt submit or queued prompt

I would keep slash-command dispatch unchanged unless maintainers prefer this option to cover all composer dispatch paths.

Why

Submitting from Insert mode and immediately continuing to type currently requires entering Insert mode again after every turn. Preserving mode would make Vim mode less disruptive while keeping existing behavior as the default.

Notes

I noticed the contributing guide says unsolicited PRs are not accepted, so I am filing this as an issue/proposal rather than opening a PR.