daemon: keep named automation in reusable background tabs - #618

Follow-up to #616.

Named local/CDP daemons now get a dedicated tab so parallel names do not navigate the same page. The lifecycle stays intentionally simple:

• First attach: create one dedicated background tab.
• Stale session: reattach to the current selected tab if it still exists, otherwise the dedicated tab.
• If the user closed both: create one background replacement. Concurrent recovery shares that replacement and delayed requests retry on the recovered session.
• Daemon shutdown or disconnect: leave every working/user tab open.

Normal helper behavior is background-first too:

• new_tab() creates a background target.
• switch_tab(target) attaches and moves the horse marker without changing Chrome's visible tab.
• switch_tab(target, activate=True) or activate_tab(target) is the explicit opt-in for a visible switch.
• Static screenshots and normal CDP input work while the tab is hidden. Activation remains available only for a user-requested visible switch or a page that demonstrably pauses visibility-dependent rendering.

This change does not automatically close normal browser tabs. Users and agents can still close a tab explicitly with close_tab().

Default daemons and named Browser Use Cloud daemons keep their existing first-page attach behavior. The existing cleanup for a harness-opened chrome://inspect permission tab also remains.

Verification:

• uv run --with pytest pytest -q — 140 passed
• git diff --check — passed
• Live macOS Chrome — a document.hidden === true background tab accepted typing and a coordinate click, captured a 3456×1730 PNG, and did not change the foreground application.
• Review race reproduction — when one request recovered the stale session first, the delayed request retried successfully on the replacement session; explicit stale-session requests still return their error.
• Shutdown regression — the real serve stop path leaves both the dedicated and user-selected tabs open.

Summary by cubic

Keeps named local daemons on a dedicated reusable background tab and stops changing the user’s visible tab. Previously we attached to the first page and switch_tab() activated it; now switch_tab() attaches without activating, stale-session recovery is scoped and serialized with tab switches, and recovered sessions publish before domain enable so delayed requests hit the right tab.

• Named local daemons create one background about:blank tab; default and named remote daemons still attach to the first real page.
• Reattach prefers the current selected tab, then the dedicated tab; if both are closed, one background replacement is created and shared by concurrent recoveries; attach/setup failures leave the created tab for retry.
• Recovery: implicit stale requests reattach and retry only on the specific recovered session; explicit session_id requests return an error; we publish the replacement before domain setup and preserve chains; tab switches wait for in-flight recovery so recovered actions are not redirected.
• Helpers: new_tab() creates background targets; switch_tab(target) attaches and moves the horse marker; activate_tab(target) opts into a visible switch; screenshots and normal CDP input work while hidden; shutdown leaves working and user tabs open.
• Migration: Call activate_tab(target) or switch_tab(..., activate=True) when a visible tab switch is required.

^{Written for commit f4c6ca2. Summary will update on new commits.}