fix: disque via statvfs() — valeurs identiques à df

Remplace sysinfo::Disks par un appel direct à libc::statvfs("/").
- used  = (f_blocks − f_bfree) × f_frsize  → correspond à df "Utilisé"
- free  = f_bavail × f_frsize              → correspond à df "Dispo"
- total = f_blocks × f_frsize

Avant (sysinfo) : used comptait les blocs réservés root → surestimation de ~3-4 Go.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent/src/metrics/disk.rs

@@ -1,34 +1,36 @@
-use sysinfo::Disks;
+use std::mem::MaybeUninit;
 
-pub fn get(disks: &Disks) -> (u64, u64, u64) {
-    for disk in disks.list() {
-        let mount = disk.mount_point().to_string_lossy();
-        if mount == "/" {
-            let total = disk.total_space();
-            let free = disk.available_space();
-            let used = total.saturating_sub(free);
-            return (used, free, total);
-        }
+/// Retourne (used, free, total) en octets pour le système de fichiers racine "/".
+/// Utilise statvfs() directement pour correspondre exactement aux chiffres de `df` :
+///   - total = f_blocks × f_frsize
+///   - used  = (f_blocks − f_bfree) × f_frsize   (blocs effectivement écrits)
+///   - free  = f_bavail × f_frsize                (disponible pour utilisateurs non-root)
+pub fn get() -> (u64, u64, u64) {
+    let path = b"/\0";
+    let mut stat = MaybeUninit::<libc::statvfs>::uninit();
+    let ret = unsafe { libc::statvfs(path.as_ptr() as *const libc::c_char, stat.as_mut_ptr()) };
+    if ret != 0 {
+        return (0, 0, 0);
     }
-    if let Some(disk) = disks.list().first() {
-        let total = disk.total_space();
-        let free = disk.available_space();
-        return (total.saturating_sub(free), free, total);
-    }
-    (0, 0, 0)
+    let stat = unsafe { stat.assume_init() };
+    let bsize = stat.f_frsize as u64;
+    let total = stat.f_blocks.saturating_mul(bsize);
+    let used  = stat.f_blocks.saturating_sub(stat.f_bfree).saturating_mul(bsize);
+    let free  = stat.f_bavail.saturating_mul(bsize);
+    (used, free, total)
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
-    use sysinfo::Disks;
 
     #[test]
     fn test_disk_coherent() {
-        let disks = Disks::new_with_refreshed_list();
-        let (used, free, total) = get(&disks);
+        let (used, free, total) = get();
+        eprintln!("résultat statvfs : used={used} free={free} total={total}");
         if total > 0 {
-            assert!(used + free <= total + 1024, "used + free > total");
+            assert!(used + free <= total + 1024 * 1024, "used + free > total");
+            assert!(total > 0, "total doit être > 0");
         }
     }
 }