Browser: agent tabs join the panel I'm viewing, and the agent and I can share control

[praneeth-red]

Problem

1. When an agent opens a new browser tab, it pops up the floating mini player, even when I already have the browser panel open on another tab. I'd rather it appear as a tab in the panel I'm looking at.
2. Only one of us can control the browser at a time, so I have to take or release control before the agent can carry on.

Proposed solution

1. If the browser panel is open, add the agent's new tab there and switch to it, instead of floating it.
2. Let my clicks and typing work while the agent is using the tab, with the agent pausing while I'm typing.

Smallest useful scope

Point 1 alone.

[Luzivog]

+1. A data point from running this locally: we patched our build (a runtime patch of SessionControl plus the stream client) so a person can click and type in an agent's server tab with no Take control step, and the agent's actions are never refused while the person uses the tab. What turned out to matter:

• Human input needs its own queue. human() goes through the same FIFO as agent actions, so the person's input froze for seconds during an agent navigation (related: [Bug]: Streamed browser hover and clicks lag behind stale queued pointer events #17538). A separate per-tab serial queue keeps their keydown/keyup order without waiting behind the agent.
• Ownership changes must not bump the generation. changeOwner() invalidates the agent's snapshot refs, including the auto-take when a person opens their own tab. With shared control, only close() needs to.
• Watching shouldn't act. A viewer that only watches an agent tab shouldn't send resize (it flips responsive sites to a narrow layout mid-task) or become the owner. Only real input (click, key, text, a dialog answer, navigation) makes it the owner. Hover and scroll don't. That way downloads, file pickers, clipboard and popups still go to the person actually using the tab.
• The inline checks need to ignore the person. runOperation's close/dialog checks (controller !== null) and tabOwner() otherwise still refuse the agent with "A human controls this tab".

In daily use it removed the most common friction: forgetting to release control and leaving the agent blocked. #18042 and the closed #17081 describe the same idea.