[flexdinesh]

• Behavior owner and resulting change:
  • use-sidebar.ts/ActivityBar.tsx: Repos is the first rail view and the initial panel; Diff is second.
  • ProjectsNav.tsx: choosing a stream in Repos opens it with preserveNavigation, so only the review updates; Repos stays selected (mobile drawer stays open). Picker, header menu and "View latest" still switch to Diff.
  • NewChangesButton: the "N new" header action now opens a "New changes" menu (shared DropdownMenu) listing unread heads in activity order. Rows show repo + age, branch + changed-file count, and linked worktree name or "Primary checkout". Choosing one opens that stream and clears its mark. Previously it silently opened the most active head, so you could not tell which diff you were opening.
  • use-modal-drawer.ts: keeps whatever main/header/rail is current inert. Needed because the drawer now stays open while opening a review remounts <main>, which previously escaped inert.
• Contracts/invariants affected; local/remote implications: frontend only; seen-state ownership unchanged; both modes covered by the new-changes browser test.
• Validation results; remaining risks:
  • mise run check: build, static, generated, conformance, contracts, JS unit, 88/88 browser, release tests pass. All Go packages pass except cmd/diffx, which hangs in TestCollectorOptionsRejectAmbiguousInputsBeforeStartup/change on origin/main too (no Go changes here); fixed in a separate PR.
  • Browser tests now open Diff explicitly where they need the file tree (matches the new user flow).
  • Risk: users who relied on Diff opening first now need one click; view selection is not persisted.
• Intentional architecture/design changes and rationale: DESIGN.md and docs/development.md updated for rail order, Repos selection, mobile drawer and the new-changes menu. contextWorktreeName shares the worktree-name rule with the picker label.
• UI evidence: new-changes menu checked in light, dark and 390px mobile (keyboard open/select, bounds within viewport); Impeccable detector clean; no Pierre geometry changes.