feat: Add disk usage percentage and warn on high usage

Add `usage_percent` field to disk collection output. When a disk
exceeds 85% usage, log at WARN level instead of INFO so it stands
out in log aggregation systems.

Author: lfrancke

src/system_information/disk.rs

@@ -6,6 +6,7 @@ pub struct Disk {
     pub mount_point: String,
     pub total_space: u64,
     pub available_space: u64,
+    pub usage_percent: f64,
 }
 
 impl Disk {
@@ -21,19 +22,41 @@ impl Disk {
 
 impl From<&sysinfo::Disk> for Disk {
     fn from(sysinfo_disk: &sysinfo::Disk) -> Self {
+        let total_space = sysinfo_disk.total_space();
+        let available_space = sysinfo_disk.available_space();
+        let usage_percent = if total_space > 0 {
+            (total_space - available_space) as f64 / total_space as f64 * 100.0
+        } else {
+            0.0
+        };
+
         let disk = Disk {
             name: sysinfo_disk.name().to_string_lossy().into_owned(),
             mount_point: sysinfo_disk.mount_point().to_string_lossy().into_owned(),
-            total_space: sysinfo_disk.total_space(),
-            available_space: sysinfo_disk.available_space(),
+            total_space,
+            available_space,
+            usage_percent,
         };
-        tracing::info!(
-            disk.mount_point,
-            disk.name,
-            disk.space.total = disk.total_space,
-            disk.space.available = disk.available_space,
-            "found disk"
-        );
+
+        if usage_percent >= 85.0 {
+            tracing::warn!(
+                disk.mount_point,
+                disk.name,
+                disk.space.total = disk.total_space,
+                disk.space.available = disk.available_space,
+                disk.space.usage_percent = format!("{:.1}%", disk.usage_percent),
+                "disk usage high"
+            );
+        } else {
+            tracing::info!(
+                disk.mount_point,
+                disk.name,
+                disk.space.total = disk.total_space,
+                disk.space.available = disk.available_space,
+                disk.space.usage_percent = format!("{:.1}%", disk.usage_percent),
+                "found disk"
+            );
+        }
         disk
     }
 }