Feature Request: Provide an `openspec diff` command to show differences between a spec and its modified version.

[soc221b]

Provide an openspec diff command or a similar feature to show differences between a spec and its modified version.

Currently, we can manually copy /openspec/changes/*/specs/A.md and paste it into /openspec/specs/A/spec.md to compare it with the existing spec.

It would be very useful to have a built-in diff feature to automatically highlight these differences.

Thanks for this powerful tool! ❤️

[TabishB]

@soc221b We had this in a very early version, but removed since the diff wasn't very readable inside of the terminal.

We're currently planning to update the workflow a bit since agents usually struggle to produce and archive the spec deltas currently. We'll revisit this as part of that.