Fix disk collector to use dynamic device detection

- Remove underlying_devices field from FilesystemConfig
- Add device detection at startup using findmnt command
- Store detected devices in HashMap for reuse during collection
- Keep all existing functionality (StoragePool, DriveInfo, SMART data)
- Detect devices only once at initialization, not every collection cycle
- Fixes agent startup failure due to missing underlying_devices config

agent/src/config/mod.rs

@@ -86,7 +86,6 @@ pub struct FilesystemConfig {
     pub fs_type: String, // "ext4", "zfs", "xfs", "mergerfs", "btrfs"
     pub monitor: bool,
     pub storage_type: String, // "single", "raid", "mirror", "mergerfs", "zfs"
-    pub underlying_devices: Vec<String>, // ["sda", "sdb", "sdc"] or ["nvme0n1"]
 }