feat: add ability to disable signing git commits

[bedecarroll]

Adding this flag for two reasons:

1. If aider is writing the code is it really something you should sign? I feel the user should make that call.
2. For those of us who use Yubikeys it can be non-obvious as to what the "wait" is when aider has finished a prompt.

I had to modify scripts/update-docs.sh slightly as there were API keys I didn't have but I did not commit the changes.

I used aider to write the unit test checking the change. The command I ran was: uv run -- aider --git-commit-no-sign --read aider/args.py --read aider/repo.py and the prompt was I've added the flag '--git-commit-no-sign' to the code base. Please update the unit tests as per the '--git-commit-verify' flag.

[CLAassistant]

All committers have signed the CLA.

[peterhadlaw]

Isn't the point of signing the git commit, to mark this commit as authored by "you". If you are writing the code (through the aider tool), you are in fact still responsible for its output.

[bedecarroll]

Yes but I really think the user should control that via the flag so they can determine the semantics.