undo: prevent restoring to root operation

[VedantMadane]

Fixes #8153

When there is only one non-root operation in history, jj undo would restore to the root operation which has no working copy, leaving the repo in an unusable state (many commands fail with "This command requires a working copy").

This prevents jj undo from restoring to the root operation and shows a helpful error message with a hint to use jj redo.

I read the discussion on the issue @martinvonz suggested warning when the current workspace no longer exists (in cli_util.rs). The current implementation takes the simpler approach of blocking undo to root, but happy to rework this to check workspace existence instead if that's preferred.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

Assisted-by: Cursor

[martinvonz]

Did you read the comments on the issue you linked to? Could you also fill out our PR template, especially since you said it's made with Cursor? Thanks

[PhilipMetzger]

Made with Cursor

I think we should use Assisted-By: <Tool> like other projects for such attributions. Also you clearly didn't read the contributing guidelines from your title, since we in general don't use fix: except for commits related to the aptly named command.