Add Amazon Bedrock AgentCore Browser support

### Description of the feature request:

Add support for Amazon Bedrock AgentCore Browser as a backend environment, enabling cloud-based browser automation with enhanced security and scalability features.

### What problem are you trying to solve with this feature?

AgentCore Browser provides a managed, serverless browser environment that offers:
- Cloud-based browser execution without local resource requirements
- Built-in session recording for debugging and compliance
- Browser signing for reduced CAPTCHA challenges via cryptographic agent identification
- Automatic scaling and resource management
- Integration with AWS security and IAM

### Any other information you'd like to share?

Add AgentCore as a third environment option alongside Playwright and Browserbase:

### Core Features
- AgentCoreComputer class with CDP connection to AgentCore Browser
- Browser signing enabled for cryptographic agent identification
- Session recording to S3 with configurable bucket and prefix
- Auto-created IAM roles with minimal scoped permissions
- Region detection with fallback chain
- Browser reuse logic to avoid duplicate browser creation

### CLI Arguments
- `--env=agentcore`: Use AgentCore browser backend
- `--recording_bucket`: S3 bucket for session recordings
- `--recording_prefix`: S3 prefix for recordings (default: "recordings")
- `--execution_role_arn`: IAM role ARN for browser execution
- `--create_execution_role`: Auto-create IAM role if needed
- `--browser_identifier`: Browser identifier (default: "aws.browser.v1")

### Dependencies
- boto3>=1.40.64
- bedrock-agentcore>=1.0.5

## Implementation
A complete implementation is available in PR #78

## Benefits
- No local browser installation required
- Serverless scaling for multiple concurrent sessions
- Built-in session replay for debugging
- Reduced CAPTCHA challenges with browser signing
- AWS-native security and compliance features

## Related
- AWS Blog: [Reduce CAPTCHAs for AI agents browsing the web with Web Bot Auth](https://aws.amazon.com/blogs/machine-learning/reduce-captchas-for-ai-agents-browsing-the-web-with-web-bot-auth-preview-in-amazon-bedrock-agentcore-browser/)