docs: AGENTS.md - remove custom depot-tools commands (#6893)

`AGENTS.md` recommends using the `git new-branch` command, which is not
available in standard git. When it's not available - agents tend to do
something like this:

```txt
> Now let me create a new branch and commit.

$ git new-branch dev/sg/XXX
Command exited with code 1                                                                                                               

> The git new-branch command doesn't exist. Let me just use standard git commands.

$ git checkout -b dev/sg/XXX
```

This patch simply replaces new-branch with `git checkout -b` in the
agent instructions.