feat: add CLAUDE.md for AI-assisted development

Adds a minimal CLAUDE.md covering project architecture, build system,
code conventions, testing, security considerations, and CI workflows.
Provides Claude Code with the context needed to assist with builds,
debugging, orchestrator work, and contributions.

Closes #801

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

CLAUDE.md

@@ -0,0 +1,80 @@
+# Unity-Builder
+
+GitHub Action and CLI that builds Unity projects for multiple platforms. Part of the [GameCI](https://game.ci) project.
+
+## Quick Reference
+
+```bash
+yarn              # install dependencies
+yarn build        # full build: tsc → ncc bundle (src/ → lib/ → dist/index.js)
+yarn test         # run all tests (jest)
+yarn test:ci      # run tests in CI mode (single-threaded, 2min timeout)
+yarn lint         # prettier + eslint check
+yarn format       # auto-format with prettier
+```
+
+## Architecture
+
+**Entry point:** `src/index.ts` → decides between CLI mode and GitHub Action mode.
+
+**Two execution paths:**
+1. **Local builds** — Docker container or native macOS (`src/model/docker.ts`, `src/model/mac-builder.ts`)
+2. **Orchestrator builds** — Remote execution on AWS ECS, Kubernetes, or other providers (`src/model/orchestrator/`)
+
+**Key modules:**
+
+| Path | Purpose |
+|---|---|
+| `src/model/build-parameters.ts` | Central config object — all build settings flow through here |
+| `src/model/input.ts` | Input resolution with priority: Action inputs → CLI flags → env override → env vars |
+| `src/model/orchestrator/orchestrator.ts` | Remote build orchestration — provider selection, workflow execution |
+| `src/model/orchestrator/providers/` | Provider plugin system (AWS, K8s, Docker, Local, Test) |
+| `src/model/orchestrator/remote-client/` | Code that runs inside remote containers (caching, hooks, artifacts) |
+| `src/model/orchestrator/workflows/` | Build workflow types (standard, custom, async) |
+| `src/model/orchestrator/services/` | Logging, locking, resource tracking |
+| `src/model/cli/` | CLI mode using commander — dispatches to `@CliFunction`-decorated methods |
+| `action.yml` | GitHub Action manifest — all inputs/outputs defined here |
+| `dist/index.js` | Bundled output (committed to repo, used by action.yml at runtime) |
+
+**Provider interface:** All providers implement `ProviderInterface` (`providers/provider-interface.ts`) with methods: `setupWorkflow`, `runTaskInWorkflow`, `cleanupWorkflow`, `garbageCollect`, `listResources`, `listWorkflow`, `watchWorkflow`.
+
+**Provider loading:** Providers can be built-in, loaded from npm, cloned from GitHub repos, or loaded from local paths (`provider-loader.ts`).
+
+## Build System
+
+The build pipeline is: `yarn` → `tsc` (src/ → lib/) → `ncc build lib` (lib/ → dist/index.js).
+
+- **dist/ is committed** — GitHub Actions loads `dist/index.js` directly, no install step on runners
+- **Pre-commit hooks** (lefthook) auto-run formatting, linting, related tests, and `yarn build` to keep dist/ in sync
+- Runtime: Node 20 (configured via Volta and action.yml `runs.using: node20`)
+
+## Code Conventions
+
+- **Files:** kebab-case (enforced by eslint `unicorn/filename-case`)
+- **Code:** camelCase variables/functions, PascalCase classes/types
+- **Formatting:** Prettier — 120 char width, single quotes, trailing commas, semicolons
+- **Linting:** ESLint with unicorn, github, prettier, jest plugins
+- **TypeScript:** strict mode, ES2020 target, CommonJS modules, experimental decorators enabled
+- **Blank line before return statements** (enforced)
+- **Blank line before block/line comments** (enforced)
+- **No `for...in` loops** — use `for...of`
+
+## Testing
+
+- **Framework:** Jest 27 with ts-jest
+- **Pattern:** `**/*.test.ts` files colocated with source
+- **Orchestrator tests:** Concentrated in `src/model/orchestrator/tests/`
+- **Run specific tests:** `yarn test -t "pattern"` or `yarn jest path/to/file.test.ts`
+- **Orchestrator integration tests** require `orchestratorTests=true` env var: `cross-env orchestratorTests=true yarn test -i -t "orchestrator"`
+
+## Security
+
+- **Never log, output, or hardcode credentials** — cloud provider secrets (AWS, GCP, K8s), Unity serial keys, keystores, and private tokens must stay in secret inputs
+- **Input validation matters** — user-supplied hook commands and custom parameters can be injection vectors; use `shell-quote` for shell escaping
+- **Keystore/license data** is base64-encoded in inputs and written to temp files at build time
+
+## CI Workflows
+
+- `integrity-check.yml` — lint, test, build on every push/PR
+- `build-tests-{ubuntu,windows,mac}.yml` — matrix builds across Unity versions and platforms
+- `orchestrator-integrity.yml` / `orchestrator-async-checks.yml` — orchestrator-specific validation