TUI Vim mode: allow starting in Insert mode by default

What variant of Codex are you using?

CLI

What feature would you like to see?

Please add an option for Vim mode to start in Insert mode by default, while still allowing users to press Esc to enter Normal mode.

Currently, enabling Vim mode permanently starts the composer in Normal mode. I would prefer a config option such as:

[tui]
vim_mode_default = true
vim_mode_initial_state = "insert"

or similar.

Motivation

Most TUI interactions begin with writing a new prompt, and only optionally editing that prompt afterward. Starting in Insert mode matches that common flow better while preserving Vim-style editing when needed.
This would also match the behavior of “vi mode” in shells like Bash: the prompt starts ready for text input, and users can press Esc to switch into Normal mode for editing.

Related issue: #21804

Additional information

_No response_