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

README.md

@@ -463,6 +463,7 @@ This is useful for multimodal AI models that can reason about visual layout, unl
 | `--headed` | Show browser window (not headless) |
 | `--cdp <port\|url>` | Connect via Chrome DevTools Protocol (port or WebSocket URL) |
 | `--auto-connect` | Auto-discover and connect to running Chrome (or `AGENT_BROWSER_AUTO_CONNECT` env) |
+| `--color-scheme <scheme>` | Color scheme: `dark`, `light`, `no-preference` (or `AGENT_BROWSER_COLOR_SCHEME` env) |
 | `--config <path>` | Use a custom config file (or `AGENT_BROWSER_CONFIG` env) |
 | `--debug` | Debug output |