Feature/undo delete

[alexesba]

Summary

Adds undo for deletes so removing a task (or a bulk selection) is no longer
irreversible. Press u in the pad to restore the most recently deleted task(s)
to their original positions.

• Single & bulk undo — works for dd and visual-mode V + d. Removed
  tasks are recorded with their original indices and re-inserted in place.
• Sequential, one transaction at a time — each delete is its own undo step;
  repeated u walks back through the history.
• Bounded & in-memory — the undo stack is capped at 25 entries and is not
  persisted across sessions (keeps behavior simple and predictable).
• Configurable mapping — mappings.undo (default u), set to false to
  disable.
• Notifies Restored N task(s) or Nothing to undo.

Changes

• store.lua: record deletions (single + bulk) onto a bounded undo stack; add
  store.undo() that restores the most recent deletion.
• actions.lua: actions.undo() with notifications; bound to the new
  mappings.undo key in the pad.
• config.lua: new mappings.undo = "u" (typed in PinPadMappings).
• Docs: README feature list, mappings table, and config block; SPEC mappings
  table.
• Tests: tests/undo_spec.lua covering single/bulk restore-to-position,
  empty-stack, the 25-deep cap, and sequential undo.

Test plan

• make test green locally (18 passed, 0 failed)
• CI runs the suite on Neovim stable + nightly
• Manual: delete a task with dd, press u, confirm it returns to its spot
• Manual: V select several tasks, d to bulk-delete, u restores all
• Manual: press u with nothing to undo → "Nothing to undo"