docs: add CLAUDE.md for Claude Code guidance

Author: libhide

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.
+
+## Build Commands
+
+```sh
+npm ci                    # Install dependencies
+npm run build             # Build both Chrome and Firefox extensions
+npm run build-chrome      # Build Chrome extension only
+npm run build-firefox     # Build Firefox extension only
+npm run watch-chrome      # Watch mode for Chrome development
+npm run watch-firefox     # Watch mode for Firefox development
+npm run format            # Format code with Biome
+npm run lint              # Check code with Biome
+npm test                  # Run linter (alias for npm run lint)
+```
+
+Build outputs go to `built/kagi_{browser}_{version}.zip`.
+
+## Architecture
+
+This is a monorepo containing Kagi Search browser extensions for Chrome, Firefox, and Safari.
+
+### Code Sharing Model
+
+- **`shared/`** - Common code used by both Chrome and Firefox extensions
+  - `src/background.js` - Service worker: session token management, auth header injection, context menus
+  - `src/popup.js` - Extension popup UI and settings management
+  - `src/summarize_result.js` - Universal Summarizer feature
+  - `src/lib/utils.js` - Shared utilities (API calls, storage, permissions)
+  - `icons/` - Extension icons
+- **`chrome/`** - Chrome-specific manifest only
+- **`firefox/`** - Firefox-specific manifest only
+- **`safari/`** - Separate Swift-based implementations (not part of JS build)
+
+The build script (`build.js`) combines `shared/*` with browser-specific manifests into zip files.
+
+### Safari Extensions
+
+Safari extensions are Swift-based native apps, separate from the npm build system. Built with Xcode.
+
+- **`safari/Universal/`** - Universal extension for iOS + macOS (MV3). Still under development - background scripts are unreliable and may be killed or fail to load.
+- **`safari/Legacy iOS/`** - Legacy iOS implementation
+- **`safari/Legacy macOS/`** - Legacy macOS implementation (minimum macOS 10.14)
+
+The iOS extension is published at: https://apps.apple.com/us/app/kagi-search-for-safari/id1607766153
+
+### Browser Detection
+
+```javascript
+IS_CHROME = typeof browser.runtime.getBrowserInfo !== 'function'
+```
+
+### Key APIs
+
+- `kagi.com/api/v0/summarize` - API-based summarization
+- `kagisuggest.com` - Search autocomplete
+
+## Code Style
+
+- Biome for linting/formatting (2-space indent, single quotes)
+- Safari code is excluded from linting
+- Node 20.x, npm 10.x required
+
+## Contributing
+
+Bugfixes and improvements are welcome. New features require prior approval via the issue tracker.