docs: add contributing guide and changelog

Author: unadlib

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on Keep a Changelog.
+
+## [0.1.0] - 2026-02-18
+
+### Added
+
+- Initial release of `@fictjs/react`.
+- Eager and resumable React island APIs: `reactify`, `ReactIsland`, `reactify$`.
+- Static loader API: `installReactIslands`.
+- React action bridging APIs: `reactAction$`, `reactActionFromQrl`.
+- Vite preset export: `fictReactPreset`.
+- CI pipelines for verification, E2E, and npm publish with provenance.

CONTRIBUTING.md

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+# Contributing
+
+Thanks for contributing to `@fictjs/react`.
+
+## Development Setup
+
+1. Use Node.js 20+.
+2. Enable Corepack and install dependencies:
+   - `corepack enable`
+   - `pnpm install --ignore-workspace --lockfile-dir .`
+
+## Local Validation
+
+Run these before opening a PR:
+
+- `pnpm lint`
+- `pnpm typecheck`
+- `pnpm test`
+- `pnpm test:it`
+- `pnpm build`
+
+Optional browser checks:
+
+- `pnpm test:e2e`
+
+## Commit and PR Guidelines
+
+- Keep commits focused and small.
+- Use clear commit messages.
+- Include tests for behavior changes.
+- Update docs when public APIs or workflows change.
+
+## Release Notes
+
+- This package is published from GitHub Actions via `.github/workflows/npm-publish.yml`.
+- npm publish uses provenance attestations.