Add persistent ChromeDriver support for faster WebDriver startup

When webdriver_start is called, now checks if chromedriver is already
running on the configured port and reuses it instead of spawning a new
process. This significantly reduces startup time for subsequent sessions.

New config option:
  [webdriver]
  persistent_chrome = true  # Keep chromedriver running between sessions

When enabled, webdriver_quit closes the browser session but leaves
chromedriver running for reuse by the next session.

crates/g3-config/src/lib.rs

@@ -179,6 +179,10 @@ pub struct WebDriverConfig {
     pub chromedriver_binary: Option<String>,
     #[serde(default)]
     pub browser: WebDriverBrowser,
+    #[serde(default)]
+    /// Keep chromedriver running after session ends for faster subsequent startups
+    /// When true, chromedriver process is not killed on webdriver_quit
+    pub persistent_chrome: bool,
 }
 
 impl Default for AgentConfig {