Implement the 002-add-combatant feature that adds the possibility to add new combatants to an encounter

specs/002-add-combatant/quickstart.md

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+# Quickstart: Add Combatant
+
+**Feature**: 002-add-combatant
+
+## Prerequisites
+
+```bash
+pnpm install
+```
+
+## Development
+
+```bash
+pnpm test:watch                   # Watch all tests
+pnpm vitest run packages/domain/src/__tests__/add-combatant.test.ts  # Run feature tests
+pnpm --filter web dev             # Dev server at localhost:5173
+```
+
+## Merge Gate
+
+```bash
+pnpm check   # Must pass before commit (format + lint + typecheck + test)
+```
+
+## Implementation Order
+
+1. **Domain event** — Add `CombatantAdded` to `events.ts` and the `DomainEvent` union
+2. **Domain function** — Create `add-combatant.ts` with the pure `addCombatant` function
+3. **Domain exports** — Update `index.ts` to re-export new items
+4. **Domain tests** — Create `add-combatant.test.ts` with all 6 acceptance scenarios + invariant checks
+5. **Application use case** — Create `add-combatant-use-case.ts`
+6. **Application exports** — Update `index.ts` to re-export
+7. **Web hook** — Update `use-encounter.ts` to expose `addCombatant` callback
+8. **Web UI** — Update `App.tsx` with name input and add button
+
+## Key Files
+
+| File | Action | Purpose |
+|------|--------|---------|
+| `packages/domain/src/events.ts` | Edit | Add CombatantAdded event type |
+| `packages/domain/src/add-combatant.ts` | Create | Pure addCombatant function |
+| `packages/domain/src/index.ts` | Edit | Export new items |
+| `packages/domain/src/__tests__/add-combatant.test.ts` | Create | Acceptance + invariant tests |
+| `packages/application/src/add-combatant-use-case.ts` | Create | Use case orchestration |
+| `packages/application/src/index.ts` | Edit | Export new use case |
+| `apps/web/src/hooks/use-encounter.ts` | Edit | Add combatant hook callback |
+| `apps/web/src/App.tsx` | Edit | Name input + add button UI |