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/types.ts

@@ -31,6 +31,7 @@ export interface LaunchCommand extends BaseCommand {
   provider?: string;
   ignoreHTTPSErrors?: boolean;
   allowFileAccess?: boolean; // Enable file:// URL access and cross-origin file requests
+  colorScheme?: 'light' | 'dark' | 'no-preference'; // Persistent color scheme override
   // Auto-load state file for session persistence
   autoStateFilePath?: string;
 }