Add Android equivalent for Agent Activity live updates

[HorusGoul]

Context

The mobile app defines an AgentActivity Live Activity widget through expo-widgets and @expo/ui/swift-ui. It is configured in apps/mobile/app.config.ts and registered through the agent-awareness relay flow.

The current remote registration path explicitly gates Live Activity registration to iOS via Platform.OS === "ios".

Why this matters

This feature shows active T3 Code agent status outside the app and supports push-updated agent activity state. Android does not have iOS Live Activities / Dynamic Island, so parity likely means an Android-native equivalent rather than a direct port.

Possible Android equivalents:

• rich ongoing notification for active agents
• notification updates for waiting-for-approval / waiting-for-input states
• lock screen / status shade presentation where supported
• deep links back into the relevant thread
• relay registration for Android device tokens and activity/update subscriptions

Feature examples

• A running agent should be visible outside the app with project/thread/model/status.
• Waiting-for-approval or waiting-for-input states should be surfaced prominently.
• Tapping an update should deep-link to the relevant thread.

Acceptance criteria

• Define the Android product equivalent for iOS AgentActivity Live Activities.
• Extend agent-awareness registration to support Android device/update tokens as appropriate.
• Implement Android notification/channel behavior for active agent updates.
• Preserve existing iOS Live Activity behavior.
• Add tests for registration branching, payload construction, and navigation/deep-link behavior.

Current test coverage note

There is a unit test for the widget layout (apps/mobile/src/widgets/AgentActivity.test.ts) and TypeScript tests for live-activity preferences/agent-awareness flows. I did not find native iOS widget/UI tests or Android notification implementation tests.