Show diff spanning multiple checkpoints

[Juice10]

Current state

Currently when the agent completes a task spread over multiple steps to see the diff you need to:

1. Know that there is a diff button on each checkpoint
   
2. Find it in between the very verbose steps:
   
3. Click on each button, and look at all of them individually

Ideally

I can see a final diff of everything it did between prompt and final task completing.

Keep in mind that using VS Code's git diff only everything that changed regardless of prompt steps

Alternatives

GHCP does this much nicer:

And can also include it in the final description of the completed task:

[Juice10]

Related to #142 and #184 (duplicate)