Add BH_NO_ACTIVATE to stop new_tab/switch_tab from stealing OS focus

[aiba]

Problem

When driving a visible browser (e.g. an isolated debug Chrome you're
watching), every new_tab() and switch_tab() raises the Chrome window to the
OS foreground and steals keyboard focus from whatever you're typing in. Two CDP
calls cause this on macOS:

• switch_tab() → Target.activateTarget raises + focuses the window.
• new_tab() → Target.createTarget opens the tab in the foreground
  (foreground is the Mac default; createTarget takes a Mac-only background
  flag).

Change

Gate both behind an opt-in BH_NO_ACTIVATE=1 env flag. Default behavior is
unchanged — activation still happens unless the flag is set.

• switch_tab skips Target.activateTarget.
• new_tab and the daemon's no-real-pages bootstrap pass background: true
  to Target.createTarget.

The flag is read client-side, so callers just prefix runs with
BH_NO_ACTIVATE=1 — no daemon restart needed. Routing is unaffected:
set_session still makes cdp()/js() default to the controlled tab, and the
🐴 title marker still shows which tab the agent drives.

Caveat: needs a non-empty window (sentinel tab)

background: true only keeps the window backgrounded when the new tab is not
the window's only tab. If the automation window has no other real, loaded
tab, Chrome raises the window to show its first tab and background can't
prevent that:

• Window already has ≥1 real loaded tab → new_tab() opens in the background,
  no focus steal.
• Window is empty (just launched, or all tabs closed) → the first new_tab()
  foregrounds the window once.

For fully focus-free behavior, keep one real loaded "sentinel" tab open in the
automation browser (e.g. launch Chrome pointed at a static local page).
about:blank is not sufficient — it's treated as internal; the anchor must
be a real loaded page (http/https/file/data).

Relationship to #402

This supersedes #402, which gated only the switch_tab activateTarget call.
That half alone doesn't stop the steal on new_tab() — Target.createTarget
raises the window independently. Verified empirically (frontmost app via
osascript, macOS / Chrome for Testing): with activateTarget skipped but no
background flag, the first new_tab() still flips frontmost from Finder →
Chrome. This PR adds the createTarget background: true half so the flag
covers the common new_tab() path too.

Testing

• Empty window: 5/5 new_tab() calls stole focus.
• With one real sentinel tab kept open: 6/6 new_tab() clean, plus
  switch_tab / screenshot / js() / close_tab all clean.

Possible follow-up

The empty-window foreground could be eliminated entirely by having the daemon
maintain a persistent keepalive tab, making the flag self-sufficient without a
user-managed sentinel. Happy to add that here or as a follow-up if you'd prefer.

---

Summary by cubic

Adds an opt-in BH_NO_ACTIVATE flag to prevent Chrome from stealing OS focus on macOS during new_tab() and switch_tab(). Default behavior is unchanged.

• New Features

  • With BH_NO_ACTIVATE=1, switch_tab() skips Target.activateTarget.
  • new_tab() and daemon bootstrap pass background: true to Target.createTarget to open tabs in the background.
  • Flag is read client-side; no daemon restart needed.

• Migration

  • Enable by prefixing runs with BH_NO_ACTIVATE=1.
  • On an empty window, the first tab may still foreground; keep one real loaded sentinel tab open (about:blank is not sufficient).

^{Written for commit 6c7e155. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}