Add AI commit message generation

[osszoi]

What does this PR do?

Adds AI-powered commit message generation. When you're in the commit message panel, press Ctrl+G and an AI will generate a commit message based on your staged changes.

How it works

• Press c to open the commit panel
• Press Ctrl+G to generate a commit message with AI
• The AI reads your staged diff (excluding lock files and other noise), looks at your branch name, and generates a message
• You can edit the generated message before committing

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc+

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I got a lot of help by the AI, to understand, find stuff, etc. I hope that's ok. Also, this is my first contribution to open source, I've been building AI stuff for 1+ year now so I thought this was a good first thing to try, and what best than contribute to my daily driver for git stuff ❤️