#122 feat(browser): support local-only screenshots

Summary

• add attachToContext?: false to Page.captureScreenshot calls inside browser_execute
• continue returning screenshot bytes to the snippet while excluding opted-out screenshots from model attachments
• document the opt-out in the browser tool description

Test plan

• full browser-execute.test.ts against local headless Chrome: 8 passed
• bun typecheck from packages/bcode-browser
• repository pre-push typecheck (16 packages)