PutSystemSettingTool

settings put system <key> <value> — write a Settings.System value (e.g. screen_brightness, screen_off_timeout). Idempotent. Delegates to putSetting; treated as failure on non-zero exit or non-empty stderr.

Privilege: requires a working ShellBackend.

Params: key (required), value (required).

On success the result map carries success (true), namespace ("system"), key, and value.