[Feature] System controls — WiFi, Bluetooth, Volume, Brightness

## Description
Allow the agent to toggle and adjust basic phone system settings.

## Implementation
- Create `SystemModule.kt`
- Add WRITE_SETTINGS permissions
- Register tools for WiFi (toggle), Bluetooth (toggle), Volume (slider), Brightness (slider)

## Acceptance Criteria
- [ ] Agent can toggle WiFi/Bluetooth reliably
- [ ] Agent can adjust volume and brightness