tabs: back to one window per session; keep new tabs out of the user's windowHEADmaster

A window shows only one tab, so sessions sharing one hid each other's tabs: no
real input and blank screenshots for every session but the one whose tab was
active (castle-pirc-web, 2026-11-08). Time-sharing the active tab would flip tabs
under the user while they sign in for an agent, so revert 66fbd30: a session
gets its own unfocused window. Kept: chrome_tab new for a session with no window
opens one for it instead of landing in the user's focused window. The primer
and chrome_tab description now say why to reuse a tab.

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