Add Chrome headless diagnostic tool

Runs automatically when --chrome-headless flag is used, checking:
- ChromeDriver installation and PATH
- Chrome/Chromium installation
- Chrome and ChromeDriver version compatibility
- config.toml chrome_binary setting
- Chrome for Testing installation
- ChromeDriver executable permissions (macOS quarantine)

Displays a detailed report with:
- Summary of detected versions and paths
- Pass/warning/error status for each check
- Specific fix suggestions for any issues found

Users can then ask g3 to help fix any detected issues.

crates/g3-computer-control/src/lib.rs

@@ -10,6 +10,7 @@ pub mod webdriver;
 // Re-export webdriver types for convenience
 pub use webdriver::{
     chrome::ChromeDriver, safari::SafariDriver, WebDriverController, WebElement,
+    diagnostics::{run_diagnostics as run_chrome_diagnostics, ChromeDiagnosticReport, DiagnosticStatus},
 };
 
 // Re-export macax types for convenience