Fix NVMe serial display and improve pool health logic

- Fix physical drive serial number display in dashboard
- Improve pool health calculation for arrays with multiple disks
- Support proper tree symbols for multiple parity drives
- Read git commit hash from /var/lib/cm-dashboard/git-commit for Build display

agent/src/collectors/nixos.rs

@@ -83,14 +83,25 @@ impl NixOSCollector {
         std::env::var("CM_DASHBOARD_VERSION").unwrap_or_else(|_| "unknown".to_string())
     }
 
-    /// Get NixOS system generation (build) information
+    /// Get NixOS system generation (build) information from git commit
     async fn get_nixos_generation(&self) -> Option<String> {
-        match Command::new("nixos-version").output() {
-            Ok(output) => {
-                let version_str = String::from_utf8_lossy(&output.stdout);
-                Some(version_str.trim().to_string())
+        // Try to read git commit hash from file written during rebuild
+        let commit_file = "/var/lib/cm-dashboard/git-commit";
+        match fs::read_to_string(commit_file) {
+            Ok(content) => {
+                let commit_hash = content.trim();
+                if commit_hash.len() >= 7 {
+                    debug!("Found git commit hash: {}", commit_hash);
+                    Some(commit_hash.to_string())
+                } else {
+                    debug!("Git commit hash too short: {}", commit_hash);
+                    None
+                }
+            }
+            Err(e) => {
+                debug!("Failed to read git commit file {}: {}", commit_file, e);
+                None
             }
-            Err(_) => None,
         }
     }
 }