feat(cli): add git-commit-msg command for conventional commits

[cmac4603]

Add new command to generate conventional commit messages from git diffs and copy to clipboard.

$ bun run packages/opencode/src/index.ts git-commit-msg -c

█▀▀█ █▀▀█ █▀▀ █▀▀▄ █▀▀ █▀▀█ █▀▀▄ █▀▀
█░░█ █░░█ █▀▀ █░░█ █░░ █░░█ █░░█ █▀▀
▀▀▀▀ █▀▀▀ ▀▀▀ ▀  ▀ ▀▀▀ ▀▀▀▀ ▀▀▀  ▀▀▀

>  Generating conventional commit message...

@  amazon-bedrock/anthropic.claude-sonnet-4-20250514-v1:0

Generated commit message:

fix(cli): did something useful...maybe

✓ Copied to clipboard!

To use this message, run:
git commit -m "fix(cli): did something useful...maybe"

Not sure if this fits with the TUI, but I find this feature super-useful with another tool I use and wondering if it could be added with some more work/tests? Looking for any advice on current implementation and if the code owners like the idea for me to spend more time on it