Refine native sidebar visual hierarchy

[jln13x]

Summary

• simplify SYNC.md around the personal fork workflow and remove Jake-specific guidance
• strengthen project, worktree, and thread visual hierarchy in dark and light mode
• flatten populated worktrees into branch-prefixed thread rows with taller targets and worktree icons
• remove redundant worktree counts, inline archive buttons, the native empty-state label, and the Main checkout label
• preserve last-thread archive confirmation and expose explicit worktree archiving through the context menu

Validation

• vp test run apps/web/src/components/Sidebar.logic.test.ts apps/web/src/components/AppSidebarLayout.logic.test.ts apps/web/src/worktreeCleanup.test.ts
• vp check
• vp run typecheck

92 focused tests pass. The full checks pass with only the existing unrelated nested-component lint warnings.