Ready: feat: keep non-foreground tabs operable (focus-emulate + tab new --background)

[soichisumi]

Summary

• focus-emulate <on|off> — emulate a focused/visible page on the active tab so automation works on a backgrounded tab.
• tab new --background — open a new tab without foregrounding it.
• Together they let an agent open and drive a tab in the background, without raising windows or stealing the user's current tab/focus.

Why

When automating a browser that a human is also using, an agent often needs to work on a tab that is not the foreground tab. Chrome backgrounds non-foreground tabs: document.visibilityState becomes hidden and rendering / requestAnimationFrame is throttled, so clicks and visibility/focus-gated handlers silently no-op. The usual workaround — foregrounding the tab (Page.bringToFront) — steals the user's current tab, which is disruptive when a person is sharing the browser.

What

• focus-emulate <on|off>: a new command that toggles CDP Emulation.setFocusEmulationEnabled on the active tab.
• tab new --background: a new flag on tab new that creates the target with CDP background: true. The tab still becomes the daemon's active page, so subsequent commands target it.
• Unit tests for both command parsers, and docs/help updates for both.

How

• focus-emulate on sends Emulation.setFocusEmulationEnabled { enabled: true } to the active page's session, so it reports as focused + visible (and resumes rendering) even while backgrounded — without raising the window or taking OS focus. off reverts it.
• tab new --background passes background: true to Target.createTarget (built with serde_json::json!, matching the existing ad-hoc createTarget call), so the tab opens without foregrounding. No change to the typed CreateTargetParams, keeping the diff isolated.

Test plan

• cargo test --bin agent-browser — parser tests pass: test_focus_emulate_on / test_focus_emulate_off / test_focus_emulate_requires_on_or_off, test_tab_new_background / test_tab_new_background_with_url.
• Built and run against a real Chrome over CDP: a background tab reports visibilityState=hidden and requestAnimationFrame ≈ 0 ticks/s; after focus-emulate on it reports visible, hasFocus()=true, and ≈60 ticks/s. tab new --background opens a tab without foregrounding it, and the new tab is still the active page for subsequent commands.
• Docs/help updated for both new commands (README.md, docs/src/app/commands/page.mdx, skill-data/core/references/commands.md, cli/src/output.rs).

Note: this does not reintroduce scoped per-command tab dispatch (the removed --tab, #1250) — there is no per-command tab routing here; both additions act on the active tab.

[vercel]

@soichisumi is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[soichisumi]

also this is needed 🙏
Cannot upgrade agent-browser wo these features #1425 #1426 #1427