Allow combination of AI generated commit message and human supplied commit message for /commit command

[mikegehard]

Issue

Currently you can either have an one line AI generated commit message or a human supplied message.

The AI generated message does a great job of categorizing the commit and telling you what changed in the commit but does nothing to include why the change was made because that is hard for the AI to guess.

I would like the ability to use the AI generated message for the "what" but allow a human to supply the "why" via the /commit command. Adding the "why" to a commit, especially on large codebases, can help add organizational and historical content to the commit.

This "why" could also be handy if we allow AI to help us process the git log as we ask questions about the history of larger/older codebases.

For example:
/commit "this is the why of the commit" would result in a commit message of

<type>:<description> (AI generated)

this is the why of the commit

I propose adding a git config setting, append-provided-message, that would allow the user to opt in to this combination so it is backwards compatible.

Version and model info

No response