Restore visual worktree thread grouping

[jln13x]

Summary

• render one compact branch header per visible worktree instead of repeating branch names on every thread
• inset conversation rows beneath worktree headers and only show Main checkout when multiple checkout groups are visible
• preserve upstream sidebar behavior when the native sidebar feature flag is disabled
• update focused tests and the personal fork behavior inventory

Validation

• vp test apps/web/src/components/Sidebar.logic.test.ts (77 tests passed)
• vp run typecheck
• vp check (passes with 10 pre-existing warnings in unrelated files)
• live development app DOM verification for the single-main-checkout case