feat: add Kernel as cloud browser provider (#200)

Add Kernel (https://kernel.sh) as a third-party cloud browser provider,
following the same pattern as Browserbase and Browser Use integrations.

Features:
- Launch browser with `-p kernel` flag or `AGENT_BROWSER_PROVIDER=kernel`
- Configurable via environment variables:
  - KERNEL_API_KEY (required)
  - KERNEL_HEADLESS (default: false)
  - KERNEL_STEALTH (default: true)
  - KERNEL_TIMEOUT_SECONDS (default: 300)
  - KERNEL_PROFILE_NAME (optional, for persistent sessions)
- Profile find-or-create: automatically creates profile if it doesn't exist
- Profile persistence: cookies/logins saved back to profile on session close
- Uses raw fetch() calls for API communication (no SDK dependency)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -739,6 +739,40 @@ When enabled, agent-browser connects to a Browser Use cloud session instead of l
 
 Get your API key from the [Browser Use Cloud Dashboard](https://cloud.browser-use.com/settings?tab=api-keys). Free credits are available to get started, with pay-as-you-go pricing after.
 
+### Kernel
+
+[Kernel](https://www.kernel.sh) provides cloud browser infrastructure for AI agents with features like stealth mode and persistent profiles.
+
+To enable Kernel, use the `-p` flag:
+
+```bash
+export KERNEL_API_KEY="your-api-key"
+agent-browser -p kernel open https://example.com
+```
+
+Or use environment variables for CI/scripts:
+
+```bash
+export AGENT_BROWSER_PROVIDER=kernel
+export KERNEL_API_KEY="your-api-key"
+agent-browser open https://example.com
+```
+
+Optional configuration via environment variables:
+
+| Variable | Description | Default |
+|----------|-------------|---------|
+| `KERNEL_HEADLESS` | Run browser in headless mode (`true`/`false`) | `false` |
+| `KERNEL_STEALTH` | Enable stealth mode to avoid bot detection (`true`/`false`) | `true` |
+| `KERNEL_TIMEOUT_SECONDS` | Session timeout in seconds | `300` |
+| `KERNEL_PROFILE_NAME` | Browser profile name for persistent cookies/logins (created if it doesn't exist) | (none) |
+
+When enabled, agent-browser connects to a Kernel cloud session instead of launching a local browser. All commands work identically.
+
+**Profile Persistence:** When `KERNEL_PROFILE_NAME` is set, the profile will be created if it doesn't already exist. Cookies, logins, and session data are automatically saved back to the profile when the browser session ends, making them available for future sessions.
+
+Get your API key from the [Kernel Dashboard](https://dashboard.onkernel.com).
+
 ## License
 
 Apache-2.0