Phase 6: Rename PRReviewStore to PRUIStore and drop GitHubStore dependency

[jennings]

Clean up what's left after the fetched data is gone.

Changes

• Drop PRReviewStore's constructor dependency on GitHubStore — it was only used for fetches.
• Remove the seeded initialPr / initialFiles provider props. If pre-seeding is desired for zero-flash route entry, call queryClient.setQueryData(...) at the route level instead.
• Delete loadPRData / loadOverviewData / loadVersionData if any residual code remains after Phases 1-4.
• Rename the store class (still useful) — proposed: PRUIStore. Rename PRReviewProvider → PRUIProvider, usePRReviewStore → usePRUIStore, usePRReviewSelector → usePRUISelector. Update all consumers.
• Prune imports; delete useCurrentUserLoader.ts and any other now-empty hook files.

Test rewrite

src/browser/contexts/pr-review/index.test.ts today mocks GitHubStore and injects fixtures through the store. After the earlier phases most of these tests are obsolete. Rewrite as:

• Focused unit tests for the remaining store methods (UI state transitions, navigation, viewed-files toggling, drafts).
• Delete tests that exercised fetched-data setters — those are now covered by RQ hook tests + optimistic-update tests from earlier phases.
• Verify the store class no longer imports GitHubStore or any fetch modules.

Tests

• Post-rename: full test suite passes; no consumer still imports the old names.
• Store class file line count drops meaningfully (target: under 1,500 lines from ~4,355).

Part of #351

Depends on #353, #354, #355, #356, #357, #358.