Rename spec 037-undo-redo to 006-undo-redo for sequential numbering

Delete merged feature branches (005–037) that inflated the auto-increment
counter in create-new-feature.sh, and renumber the undo-redo spec to
follow the existing 001–005 sequence.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

specs/006-undo-redo/quickstart.md

@@ -0,0 +1,53 @@
+# Quickstart: Undo/Redo
+
+**Feature**: 006-undo-redo
+**Date**: 2026-03-26
+
+## Overview
+
+This feature adds undo/redo to all encounter state changes using the memento (snapshot) pattern. Each action captures the pre-action encounter state onto an undo stack. Undo restores the previous state; redo re-applies an undone state.
+
+## Implementation Layers
+
+### Domain (`packages/domain/src/undo-redo.ts`)
+
+Pure functions for stack management:
+- `pushUndo(state, snapshot)` — push snapshot, cap at 50, clear redo
+- `undo(state, currentEncounter)` — pop undo, push current to redo
+- `redo(state, currentEncounter)` — pop redo, push current to undo
+- `clearHistory()` — reset both stacks
+
+All functions take and return immutable data. No I/O.
+
+### Application (`packages/application/src/`)
+
+Use cases that orchestrate domain calls via store ports:
+- `undoUseCase(encounterStore, undoRedoStore)` — execute undo
+- `redoUseCase(encounterStore, undoRedoStore)` — execute redo
+
+New port interface `UndoRedoStore` in `ports.ts`:
+- `get(): UndoRedoState`
+- `save(state: UndoRedoState): void`
+
+### Web Adapter (`apps/web/src/`)
+
+**Hook (`use-encounter.ts`)**: Wraps every action callback to capture pre-action snapshot. Exposes `undo()`, `redo()`, `canUndo`, `canRedo`.
+
+**Persistence (`persistence/undo-redo-storage.ts`)**: Save/load undo/redo stacks to localStorage keys `"initiative:encounter:undo"` and `"initiative:encounter:redo"`.
+
+**UI (`components/turn-navigation.tsx`)**: Undo/Redo buttons in the top bar, inboard of the turn step buttons, disabled when stack is empty.
+
+**Keyboard (`hooks/use-undo-redo-shortcuts.ts`)**: Global keydown listener for Ctrl+Z / Ctrl+Shift+Z (Cmd on Mac). Suppressed when text input has focus.
+
+## Key Design Decisions
+
+1. **Memento over Command**: Full encounter snapshots, not inverse events. Simpler at encounter scale (~2-5 KB per snapshot).
+2. **Capture in hook, not domain**: Snapshot capture happens in the adapter layer. Domain and application layers are unaware of undo/redo.
+3. **React state for stacks**: Enables reactive button disabled states without manual re-render triggers.
+4. **Clear is not undoable**: Both stacks reset on encounter clear (per spec).
+
+## Testing Strategy
+
+- **Domain tests**: Pure function tests for stack operations (push, pop, cap, clear, undo/redo roundtrip).
+- **Application tests**: Use case tests with mock stores.
+- **Integration**: Spec acceptance scenarios mapped to test cases (undo restores state, redo reapplies, new action clears redo, keyboard suppression during input focus).