Add contributing guide and CI badge

CONTRIBUTING.md

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+# Contributing
+
+Thanks for your interest in contributing to agent-browser. This guide covers local setup, style expectations, and the pull request workflow.
+
+## Development setup
+
+1. Install dependencies:
+
+```bash
+pnpm install
+```
+
+2. Build the project:
+
+```bash
+pnpm build
+```
+
+3. Build the native CLI (requires Rust):
+
+```bash
+pnpm build:native
+```
+
+### Rust tests (optional)
+
+```bash
+cd cli
+cargo test
+```
+
+## Code style and conventions
+
+- Use pnpm for all package management and scripts.
+- Do not use emojis in code, output, or documentation.
+- CLI flags must use kebab-case (for example, `--auto-connect`).
+- Do not add hardcoded ANSI color codes. Use `cli/src/color.rs` for CLI colors.
+
+If you introduce user-facing features (new flags, commands, behaviors, environment variables), update the following locations:
+
+- `cli/src/output.rs` (CLI help output)
+- `README.md`
+- `skills/agent-browser/SKILL.md`
+- `docs/src/app/` documentation pages
+- Inline doc comments in relevant source files
+
+## Pull request workflow
+
+1. Create a feature branch from `main`.
+2. Keep changes focused and scoped to a single topic.
+3. Run relevant tests and formatters where applicable.
+4. If your change affects releases, add a changeset with `pnpm changeset`.
+5. Ensure CI is green before requesting review.
+
+## Reporting issues
+
+Please include clear reproduction steps, expected behavior, and environment details when filing issues.