feat(review): add jj review workflows

[graemefolk]

Summary

• add JJ as a first-class VCS backend for local review flows across the hook server, OpenCode command path, and shared review-core types
• move compare-target semantics into the VCS abstraction so the review server and UI stop branching on JJ-specific flags and instead consume provider-supplied picker metadata
• extend the review UI and feedback/export flows to support JJ diff modes, compare-target selection, and JJ-specific copy and empty states

[backnotprop]

Sounds great

[madhusudancs]

Can we please merge this? I love plannotator. It's been a big productivity unlock for me. And same goes for jj. Having the ability to combine the two will have a compounding effect. Right now, I use plannotator for all planning work and fallback to Github PR UI for code reviews. Being able to do it locally would be highly valuable.

Not an expert in plannotator codebase, but I am happy to help in anyway possible to push this through. Let me know.

[backnotprop]

@madhusudancs pushing the review through now

[graemefolk]

Changes look really clean, thanks for fixing it up!

[backnotprop]

Thanks, bear with me. Certain edge cases and things. Plus structuring for Pi users and introduction of another VCS after (git butler)

[backnotprop]

This will merge tonight though

[madhusudancs]

@backnotprop thank you so much!

[backnotprop]

Released in 0.19.11