Add CLAUDE.md with project commands, architecture, and conventions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+pnpm check              # Merge gate — must pass before every commit (format + lint + typecheck + test)
+pnpm test               # Run all tests (Vitest)
+pnpm test:watch         # Tests in watch mode
+pnpm typecheck          # tsc --build (project references)
+pnpm lint               # Biome lint
+pnpm format             # Biome format (writes)
+pnpm --filter web dev   # Vite dev server (localhost:5173)
+pnpm --filter web build # Production build
+```
+
+Run a single test file: `pnpm vitest run packages/domain/src/__tests__/advance-turn.test.ts`
+
+## Architecture
+
+Strict layered architecture with ports/adapters and enforced dependency direction:
+
+```
+apps/web (React 19 + Vite)  →  packages/application (use cases)  →  packages/domain (pure logic)
+```
+
+- **Domain** — Pure functions, no I/O, no framework imports. All state transitions are deterministic. Errors returned as values (`DomainError`), never thrown. Adapters may throw only for programmer errors.
+- **Application** — Orchestrates domain calls via port interfaces (e.g., `EncounterStore`). No business logic here.
+- **Web** — React adapter. Implements ports using hooks/state.
+
+Layer boundaries are enforced by `scripts/check-layer-boundaries.mjs`, which runs as a Vitest test. Domain and application must never import from React, Vite, or upper layers.
+
+## Conventions
+
+- **Biome 2.0** for formatting and linting (no Prettier, no ESLint). Tab indentation, 80-char lines. Imports are auto-organized alphabetically.
+- **TypeScript strict mode** with `verbatimModuleSyntax`. Use `.js` extensions in relative imports when required by the repo's ESM settings (e.g., `./types.js`).
+- **Branded types** for identity values (e.g., `CombatantId`). Prefer immutability/`readonly` where practical.
+- **Domain events** are plain data objects with a `type` discriminant — no classes.
+- **Tests** live in `packages/*/src/__tests__/*.test.ts`. Test pure functions directly; map acceptance scenarios and invariants from specs to individual `it()` blocks.
+- **Feature specs** live in `specs/<feature>/` with spec.md, plan.md, tasks.md. The project constitution is at `.specify/memory/constitution.md`.
+
+## Constitution (key principles)
+
+The constitution (`.specify/memory/constitution.md`) governs all feature work:
+
+1. **Deterministic Domain Core** — Pure state transitions only; no I/O, randomness, or clocks in domain.
+2. **Layered Architecture** — Domain → Application → Adapters. Never skip layers or reverse dependencies.
+3. **Clarification-First** — Ask before making non-trivial assumptions.
+4. **MVP Baseline** — Say "MVP baseline does not include X", never permanent bans.
+5. **Every feature begins with a spec** — Spec → Plan → Tasks → Implementation.