Implement the 003-remove-combatant feature that adds the possibility to remove a combatant from an encounter

specs/003-remove-combatant/plan.md

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+# Implementation Plan: Remove Combatant
+
+**Branch**: `003-remove-combatant` | **Date**: 2026-03-03 | **Spec**: [spec.md](./spec.md)
+**Input**: Feature specification from `/specs/003-remove-combatant/spec.md`
+
+## Summary
+
+Add a `removeCombatant` pure domain function that removes a combatant by ID from an Encounter, correctly adjusts `activeIndex` to preserve turn integrity, keeps `roundNumber` unchanged, and emits a `CombatantRemoved` event. Wire through an application-layer use case and expose via a minimal UI remove action per combatant.
+
+## Technical Context
+
+**Language/Version**: TypeScript 5.x (strict mode, verbatimModuleSyntax)
+**Primary Dependencies**: React 19, Vite
+**Storage**: In-memory React state (local-first, single-user MVP)
+**Testing**: Vitest
+**Target Platform**: Web (localhost:5173 dev, production build via Vite)
+**Project Type**: Web application (monorepo: packages/domain, packages/application, apps/web)
+**Performance Goals**: N/A (local-first, small data sets)
+**Constraints**: Domain must be pure (no I/O); layer boundaries enforced by automated script
+**Scale/Scope**: Single-user, single encounter at a time
+
+## Constitution Check
+
+*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*
+
+| Principle | Status | Evidence |
+|-----------|--------|----------|
+| I. Deterministic Domain Core | PASS | `removeCombatant` is a pure function: same input → same output, no I/O |
+| II. Layered Architecture | PASS | Domain function → use case → React hook/UI. No layer violations. |
+| III. Agent Boundary | N/A | No agent layer involved in this feature |
+| IV. Clarification-First | PASS | Spec fully specifies all activeIndex adjustment rules; no ambiguity |
+| V. Escalation Gates | PASS | All functionality is within spec scope |
+| VI. MVP Baseline Language | PASS | No permanent bans introduced |
+| VII. No Gameplay Rules | PASS | Removal is encounter management, not gameplay mechanics |
+
+**Gate result**: PASS — no violations.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/003-remove-combatant/
+├── plan.md
+├── research.md
+├── data-model.md
+├── quickstart.md
+└── tasks.md
+```
+
+### Source Code (repository root)
+
+```text
+packages/domain/src/
+├── remove-combatant.ts          # Pure domain function
+├── events.ts                    # Add CombatantRemoved to DomainEvent union
+├── types.ts                     # Existing types (no changes expected)
+├── index.ts                     # Re-export removeCombatant
+└── __tests__/
+    └── remove-combatant.test.ts # Acceptance scenarios from spec
+
+packages/application/src/
+├── remove-combatant-use-case.ts # Orchestrates store.get → domain → store.save
+└── index.ts                     # Re-export use case
+
+apps/web/src/
+├── hooks/use-encounter.ts       # Add removeCombatant callback
+└── App.tsx                      # Add remove button per combatant + event display
+```
+
+**Structure Decision**: Follows the existing monorepo layered architecture (packages/domain → packages/application → apps/web) exactly mirroring the addCombatant feature's file layout.