feat(extension): migrate Timer to API v2 background runtime #1

The Timer manifest describes a timer that survives closing its panel, but API v1 activates the module inside each view document. Closing the view therefore destroys the engine, commands, reminder schedule, and notification path.

Intended behavior

• One API v2 runtime owns timing, commands, wall-clock persistence, and background notifications.
• The panel is an independent subscriber that can close and reopen without resetting state.
• Contributed default-duration and theme settings affect the runtime.
• A reminder awaiting dismissal survives an Agent Code restart.
• The committed production bundle has separate runtime and view entries.

Acceptance criteria

• Engine and runtime behavior have deterministic tests for timing, pause/resume, reminders, restart recovery, settings, malformed actions, and viewless notifications.
• Agent Code installs the actual bundle and verifies panel mount, close, reopen, and countdown progress while no view exists.
• API v1 saved timer data remains readable.