Add desktop control security policy and opt-in model #461

@dataclass
class DesktopSecurityPolicy:
    # Activation
    enabled: bool = False                    # Must be explicitly enabled
    require_env_var: bool = True             # GAIA_DESKTOP_CONTROL_ENABLED=true

    # Application control
    allowed_apps: List[str] = None           # None = all (except blocked)
    blocked_apps: List[str] = [              # Never control these
        "Terminal", "iTerm", "cmd", "powershell",  # Shell access
        "System Preferences", "Settings",          # System config
        "Keychain", "Credential Manager",          # Credentials
    ]

    # Input controls
    mouse_enabled: bool = True
    keyboard_enabled: bool = True
    max_actions_per_minute: int = 60
    blocked_key_combos: List[str] = ["ctrl+alt+del", "cmd+q"]

    # Screenshot controls
    persist_screenshots: bool = False        # Don't save to disk by default
    redact_credentials: bool = True          # Blur password fields

    # Safety
    confirm_destructive: bool = True         # Confirm before closing windows
    max_session_duration_min: int = 30       # Auto-disable after 30 min
    require_user_confirmation: bool = False  # Optional: confirm each action