[agent-browser] Expose ignoreDefaultArgs option for Brave Shields

## Problem When using agent-browser with Brave as the custom browser (`executablePath`), Brave's built-in ad/tracker blocker (Shields) does not work. The Shields icon shows as active but blocks 0 requests. The root cause is a single Playwright default launch flag: `--disable-component-update`. This flag prevents Brave from downloading its ad-block filter lists, which Shields needs to function. On a fresh or incognito profile, there are no cached filter lists, so Shields has nothing to block with. This cannot be worked around by adding `--enable-component-update` via the `args` config -- Chromium does not treat these as toggles, and the disable flag always wins when both are present. ## Proposed Solution Expose Playwright's `ignoreDefaultArgs` option in agent-browser's launch config. This option accepts an array of specific default arguments to exclude. Example config for Brave with working Shields: ```json { "executablePath": "/path/to/brave", "headed": true, "ignoreDefault