Wire review history into local Codiff

[xortive]

Summary

• add the shared GitHub review-history adapter
• adopt shared walkthrough authoring in Electron and mount the shared local review host
• add separate local GitLab and GitHub history backends
• connect provider-neutral IPC, walkthrough generation, progress events, and local follow-up documentation

Stacked on #1.

Validation

• focused local provider and host tests pass
• vpr build passes at this revision
• five broad App-render assertions remain stale and also fail against the previously pushed tree

banana banana banana

[xortive]

• Wire review history into local Codiff Wire review history into local Codiff #2 👈
• Polish shared review-history UI and GitHub evolution Add shared review-history UI #1
• Centralize provider-neutral evolution projection Add Codiff Web review-history foundations #4
• Keep the walkthrough arc fixed above local content Polish existing review surfaces #3
• main

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This stack of pull requests is managed by jj-ryu.