feat(app): add the native Control Centre companion

[LMLiam]

Goal

Ship a macOS 26 companion with a stateful Control Centre toggle and no persistent Dock or menu-bar item.

Acceptance criteria

• Generate the Xcode project from project.yml.
• Embed a WidgetKit ControlWidgetToggle.
• Read active and inactive state from pmset.
• Use the same narrow helper client as the CLI.
• Provide setup, diagnostics, removal, and optional CLI exposure.
• Pass the ad-hoc-signed clean-account feasibility gate.
• Preserve discovery and setup across an app update.

Owned files

project.yml, App/ClamshellApp/**, App/ClamshellControl/**, their tests, and docs/control-feasibility-test.md.

Dependencies

Core CLI, helper, and privileged setup.

Verification

xcodegen generate
xcodebuild -project Clamshell.xcodeproj -scheme ClamshellControlTests -destination 'platform=macOS' test
xcodebuild -project Clamshell.xcodeproj -scheme ClamshellApp -configuration Release -destination 'platform=macOS' CODE_SIGN_IDENTITY=- DEVELOPMENT_TEAM= build

Plan: Phase 6, Tasks 13–15