Give the E2E agent its own isolated browser

Running browser tests from the session you are working in means the test run and your own navigation fight over one browser, and you get failures that have nothing to do with the code.

Define a dedicated subagent with its own isolated browser server in its frontmatter, and explicitly disallow the parent session's browser tools on it. The subagent then physically cannot reach into your browser, so a full e2e sweep can run while you keep working. Worth doing even solo - the isolation is what makes the runs reproducible.