add --color-scheme flag for persistent dark/light mode (#528)

Fixes #519. Playwright defaults `colorScheme` to `light` on all new contexts, overriding the browser/OS dark mode setting. This is especially disruptive in CDP mode, where every reconnection resets the scheme. The `set media dark` command also didn't persist its choice to new tabs or pages.

- Add `--color-scheme <dark|light|no-preference>` flag, config key (`colorScheme`), and env var (`AGENT_BROWSER_COLOR_SCHEME`)
- Store the preference in `BrowserManager` and automatically apply it to all new contexts (via Playwright's context option) and all new pages (via `page.emulateMedia` in `setupPageTracking`)
- `set media dark/light` now also persists its choice for subsequent pages and tabs

src/actions.ts

@@ -2069,6 +2069,9 @@ async function handleEmulateMedia(
     reducedMotion: command.reducedMotion,
     forcedColors: command.forcedColors,
   });
+  if (command.colorScheme) {
+    browser.setColorScheme(command.colorScheme);
+  }
   return successResponse(command.id, { emulated: true });
 }