docs: prepare repository for open-source collaboration

CONTRIBUTING.md

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+# Contributing to sub2api-mobile
+
+Thanks for your interest in contributing.
+
+## Development Setup
+
+1. Fork and clone the repository.
+2. Install dependencies:
+
+   ```bash
+   npm ci
+   ```
+
+3. Start the app:
+
+   ```bash
+   npm run start
+   ```
+
+4. For web + admin proxy testing (recommended for admin endpoints):
+
+   ```bash
+   SUB2API_BASE_URL="https://your-upstream-host.example.com" \
+   SUB2API_ADMIN_API_KEY="admin-xxxx" \
+   ALLOW_ORIGIN="http://localhost:8081" \
+   npm run dev:web-proxy
+   ```
+
+## Branching and Commits
+
+- Create a feature branch from `main`.
+- Keep commits focused and atomic.
+- Prefer Conventional Commit prefixes when possible (`feat:`, `fix:`, `docs:`, `chore:`).
+
+## Pull Request Guidelines
+
+Before opening a PR:
+
+- Ensure the app builds and starts.
+- Confirm no secrets are committed.
+- Update documentation when behavior or setup changes.
+- Keep screenshots up to date for visible UI changes.
+
+In your PR description, include:
+
+- What changed
+- Why it changed
+- Any risk/impact notes
+- Verification evidence (commands + results)
+
+## Coding Guidelines
+
+- Follow existing project patterns and naming conventions.
+- Keep changes minimal and avoid unrelated refactors.
+- Prefer clear, maintainable code over clever shortcuts.
+
+## Reporting Issues
+
+- Use GitHub Issues with reproduction steps.
+- Include platform details (`ios` / `android` / `web`) and environment info.
+
+## Community
+
+By participating, you agree to our [Code of Conduct](CODE_OF_CONDUCT.md).