Add Option+Tab task switcher overlay

[h0x91b]

Summary

• Adds a keyboard-driven task switcher: hold Option and tap Tab to flip through active tasks of the current project, release to jump. Option+Shift+Tab spans all projects, and Shift can be held/released live while the overlay is open to toggle scope.
• Arrows ↑↓ move both ways, Enter opens, Esc cancels, click commits. Order is most-recently-used so a quick tap-tap toggles the two latest tasks.
• Each row shows a live terminal snapshot, task title, and overview. On Linux the binding is Ctrl+Tab / Ctrl+Shift+Tab (the WM grabs Alt+Tab).
• Grounded against the UX bible (rejected a command palette as surface creep); ships as a transient task_switcher HUD overlay reusing the sidebar's task_jump semantics. Adds MRU tracking to app state, i18n (en/ru/es), tips, unit + component tests, a UX decision, and decision record 069.