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/protocol.ts

@@ -49,6 +49,7 @@ const launchSchema = baseCommandSchema.extend({
   provider: z.string().optional(),
   ignoreHTTPSErrors: z.boolean().optional(),
   allowFileAccess: z.boolean().optional(),
+  colorScheme: z.enum(['light', 'dark', 'no-preference']).optional(),
   profile: z.string().optional(),
   storageState: z.string().optional(),
 });