Vim: allow Git commit editor to start in insert mode

[liamwh]

Body

Summary

When Vim mode is enabled, I’d like the Git commit editor to optionally open in insert mode by default.

Motivation

For normal source files, starting in normal mode is what I want. But the Git commit editor is different: opening it usually means “I want to type a commit message now”.

In a keyboard-driven Git workflow, this adds a small but frequent bit of friction:

1. Open Git panel
2. Review/stage changes
3. Open the commit editor
4. Press i
5. Type the commit message

Step 4 feels unnecessary in this context.

Requested behaviour

Please add a setting to let the Git commit editor start in insert mode without changing the global Vim default mode for normal editor buffers.

Example shape:

{
  "git": {
    "commit_editor_vim_default_mode": "insert"
  }
}