feat: global fold/unfold — collapse or expand all repositories at once

[shouze]

Context

Part of the 🗂 TUI — Navigation & result management EPIC #66.

Currently the TUI only supports ← / → to fold/unfold a single repository at a time. This feature adds a single shortcut to toggle the folded state of all repositories simultaneously.

Acceptance criteria

• A single shortcut (e.g. Shift+← / Shift+→, or Z) collapses / expands all repository sections at once
• Toggling is idempotent: pressing "fold all" when all are already folded has no visible effect (or cycles to unfold)
• The cursor remains visible after the toggle (scroll offset adjusts if needed)
• Keyboard shortcut is documented in docs/reference/keyboard-shortcuts.md

Implementation notes

• Logic lives in src/tui.ts (keyboard handler) — no pure-function module change needed unless toggling state warrants a helper
• The per-repo fold state is stored in the foldedRepos set; "fold all" = add every repo key; "unfold all" = clear the set
• After toggling, call the existing cursor-visibility logic to ensure the view scrolls correctly

Branch

feat/tui-global-fold-unfold → merge into feat/tui-navigation-result-management

Validation

bun test
bun run lint
bun run format:check
bun run knip