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.

config.example.toml

@@ -80,3 +80,4 @@ model = "claude-sonnet-4-5"
 # browser = "chrome-headless"     # Default. Alternative: "safari"
 # chrome_binary = "/path/to/chrome"        # Optional: custom Chrome path
 # chromedriver_binary = "/path/to/driver"  # Optional: custom ChromeDriver path
+# persistent_chrome = true                  # Keep chromedriver running between sessions for faster startup