feat(mcp): support saving screenshots to workspace file

[pcgeek86]

Summary

Adds an optional filename argument to the MCP browser_take_screenshot tool, allowing callers to save screenshot captures directly to a file within the configured actor workspace instead of returning inline base64 image data.

Changes

• Agent (agent/src/lib.rs):
  • Extend BrowserOp::TakeScreenshot with an optional filename: Option<String>.
  • Add write_output_bytes to handle binary file output confined to the configured actor workspace.
  • Return a confirmation text message (Screenshot written to <path>.) when filename is specified.
  • Add unit test binary_output_writer_confines_relative_and_absolute_paths.
• MCP Server (mcp/src/tools.rs):
  • Update ScreenshotArgs and schema for browser_take_screenshot to accept an optional filename string property.
  • Add schema and argument parser unit tests.
• Fixtures & Tests:
  • Update mcp/tests/fixtures/tools-list.json to reflect the schema update.
  • Add E2E stdio test verifying workspace confinement and file saving.
• CLI (cli/src/session.rs):
  • Update BrowserOp::TakeScreenshot call site.