After strict isolation (v1.5.16) a session's tracked set is only its OWN tabs, so
in `adopt_newly_opened` (run after every click to follow a pop-up) EVERY foreign
tab looks "new" relative to the session's `before` set and got adopted — a
click-heavy flow on a busy shared Chrome pulled other agents'/the user's tabs
(github, Lark, iphone-use) into the session mid-flow.
A pop-up the agent itself opened can't be told apart from a foreign tab over the
relay (the synthesized targetInfo carries no opener/window/group), so on the relay
`adopt_newly_opened` now adopts nothing: the agent drives only tabs it explicitly
created; pop-ups (OAuth/login windows) are the user's. Launched browsers (every
tab ours) still follow pop-ups. Verified live: a fresh click-heavy relay session
stays clean (only its own tabs), and 5 concurrent agents churning tabs show zero
cross-agent drift.