Agent negotiation #118
Description
In the potential future described in #51, we could have in-page agent, extension-based agents and in-browser agents, all eager to serve the user.
If there's no way to decide which agent should be the agent the user works with, it's likely that all 3 will pop up. The user would then have to discard the agents they don't care about at that very moment (through e.g. a close button).
That feels cumbersome.
It'd be better if users could set their preferences (per site, global, based on time of day, or whatever else) and the different agents could declare their presence to the browser so that it'd let the user choose.