feat(core): add PowerState skeleton in new power module

Introduces crates/core/src/power.rs with the PowerState enum
(OnAc / OnBattery / Unknown) and a unit test confirming the three
variants are distinct. Registers the module in lib.rs and adds
tempfile = "3" to dev-dependencies for use in later tasks.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/core/src/lib.rs

@@ -5,6 +5,7 @@ pub mod model_registry;
 pub mod paths;
 pub mod process_watch;
 pub mod recommendation;
+pub mod power;
 pub mod types;
 
 pub use error::{MagnotiaError, Result};