fix: enter terminal-mode when opening Codex

[joris97jansen]

Description
Issue
When opening Codex from Neovim, the CLI prompt was not interactive. Enter inserted a newline instead of selecting options (e.g., the update prompt), because the buffer stayed in normal mode (mode() = n) even though it was a terminal buffer.

Fix
Ensure the Codex terminal buffer always enters terminal-job mode on open/focus. I added an auto_insert config (default true) and an autocmd on BufEnter, WinEnter, TermOpen, and TermEnter that calls startinsert for filetype=codex terminal buffers.

Result
mode() is now t when the Codex window is focused, and the update prompt accepts input and Enter as expected.

Testing

• Manual: :CodexToggle, verify update prompt accepts Enter
• :echo mode() returns t while focused in Codex window