Datasette Agent 0.4a0: browser-run tools and custom JavaScript support

Datasette Agent version 0.4a0 introduces a new capability: the await context.browser_task() mechanism, which allows Agent plugins to run code directly in the user's browser. The new feature makes it easier for Datasette Agent plugins to provide tools that execute custom JavaScript on the client side. The release notes the issue/feature under #33, and the developers say this is a significant innovation for LLM-related tool usage. The change increases plugin interactivity and enables richer user interfaces without burdening server-side logic. Security and privacy considerations are important: running client-side code can pose risks, so developers must account for sandboxing, user consent, and cross-origin restrictions. The feature is particularly useful for interactive visualizations, quick data manipulations, and immediate execution of user actions. The release appears as version 0.4a0, and further refinements and security additions are expected based on developer community feedback.