fix: log git hash on build success and startup to verify which commit is running

Writes HEAD short hash to .huskies/build_hash after successful cargo
build. Logs it on startup as [startup] Running build: <hash>. No more
guessing whether the rebuild actually deployed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/main.rs

@@ -171,6 +171,11 @@ async fn main() -> Result<(), std::io::Error> {
         }
     });
 
+    // Log the build hash so we can verify which commit is running.
+    let build_hash = std::fs::read_to_string(".huskies/build_hash")
+        .unwrap_or_else(|_| "unknown".to_string());
+    slog!("[startup] Running build: {}", build_hash.trim());
+
     let app_state = Arc::new(SessionState::default());
     let cwd = std::env::current_dir().unwrap_or_else(|_| PathBuf::from("."));
     // Migrate legacy root-level store.json into .huskies/ if the new path does