fix(screenshot): hide overlays during capture so shots only contain page content

[BB-fat]

Problem

bsk screenshot (and element screenshots via Page.captureScreenshot) capture the extension's in-page overlays — the Agent-control mask, help/record/borrow panels — because they are DOM injected by the content script (shadow host with data-bsk-overlay). Agents end up with screenshots polluted by BSK's own chrome instead of pure page content.

Fix

Suppress overlays around every capture, with a painted-frame guarantee:

1. overlay.css: new :host([data-bsk-capture-hidden]) { display: none !important; } rule — hiding the host removes the whole shadow tree from compositing.
2. Content side (content/capture-suppress.ts): on begin it sets the attribute (refcounted for concurrent captures), then waits two requestAnimationFrames before ack'ing, so the compositor has produced a frame without the overlay before any capture runs. end decrements and restores at zero. Host remounts mid-suppression keep the hidden attribute. A 500ms rAF fallback prevents deadlock on hidden tabs (rAF never fires there; the attribute is applied synchronously, so any composited frame is still overlay-free).
3. Background side (lib/capture-suppress-bridge.ts): withOverlaysHiddenForCapture(tabId, fn) — begin (await ack) → capture → finally end. If the tab has no content script (chrome:// etc.), begin fails and the capture proceeds directly (no overlay can exist there anyway).
4. tools/observation.ts: both capture paths wrapped — element clip via CDP and full-page captureVisibleTab.

No protocol/CLI changes; behaviour fix only.

Tests

• 13 new vitest cases: content-side controller (attribute set/remove, two-frame ack, refcount, stray end, host remount), bridge (begin→capture→end ordering, capture error still restores, no-content-script passthrough, end-failure swallowed), and handleScreenshot integration asserting capture happens while hidden.
• pnpm lint, tsc --noEmit, wxt build: clean.
• Full vitest: 507 passed; the 41 React.act tsx failures are identical to the base commit (pre-existing env issue).