fix(macos): reject --fullscreen for macOS desktop/menubar helper surfaces #2799

Decision

Reject --fullscreen for macOS desktop and menubar surfaces, and keep it for macOS app sessions. This decision was cross-checked with a second model and against the code at 01328de411.

Facts

• The macOS helper reads only non-app surfaces: see usesMacOsHelperSurface / usesMacOsSurfaceScreenshot in packages/platform-apple/src/interactor.ts:417-425. App sessions go through the runner, which handles --fullscreen correctly by choosing between the full display and the app window (apple/runner/AgentDeviceRunner/AgentDeviceRunnerUITests/RunnerTests+CommandExecution.swift:2058). Don't change that path.
• For desktop and menubar surfaces, the helper parses --fullscreen (apple/macos-helper/Sources/AgentDeviceMacOSHelper/main.swift:439), ignores it (:550-551 _ = fullscreen), always captures the main display (:557-566), and echoes the flag back (:441, ScreenshotResponse.fullscreen at :78). TS sends the flag (packages/platform-apple/src/os/macos/helper.ts:476-485) and reports it (core/screenshot.ts:157,169,234).
• This has been the behavior since feat: close macOS parity gaps #311 (876f091, v0.11.2).

Required behavior

• On macOS, a screenshot with an explicit fullscreen and surface set to desktop or menubar fails with INVALID_ARGS, saying these surfaces always capture the main display. It fails before any capture. Without fullscreen, the behavior is unchanged.
• macOS app sessions and every other platform keep accepting --fullscreen unchanged.
• Remove --fullscreen from the helper: the argument parse, the parameter, and the fullscreen response field. Remove it from the TS helper argv and from the result metadata for helper surfaces. The reported metadata must describe what was actually captured, the main display.
• Update the surface-specific help text for screenshot.

Completion conditions

• TS routing tests cover desktop and menubar with fullscreen (refused), desktop without it (helper argv has no --fullscreen), and a macOS app session with fullscreen (runner path, unchanged).
• A helper Swift test in apple/macos-helper asserts the argument is gone from the response.
• pnpm check:macos-helper, pnpm check:affected --run and pnpm check:command-docs are green.
• Live check on a Mac: a desktop capture, an app session with --fullscreen and one without, where the with and without dimensions differ. Record the commands and dimensions in the PR.