Make Chrome headless the default WebDriver browser

- Add --safari flag to CLI for explicitly choosing Safari
- Update --chrome-headless flag description to indicate it's the default
- Update README to reflect Chrome headless as default
- Remove broken link to non-existent docs/webdriver-setup.md
- Add Safari flag handling in all webdriver config locations

The config already had ChromeHeadless as the default, this commit
updates the CLI and documentation to match.

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

@@ -8,7 +8,9 @@ pub mod types;
 pub mod webdriver;
 
 // Re-export webdriver types for convenience
-pub use webdriver::{safari::SafariDriver, WebDriverController, WebElement};
+pub use webdriver::{
+    chrome::ChromeDriver, safari::SafariDriver, WebDriverController, WebElement,
+};
 
 // Re-export macax types for convenience
 pub use macax::{AXApplication, AXElement, MacAxController};