Make disk collector filesystems field optional for auto-discovery

Allow agent configuration without explicit filesystems list by making
the field optional with serde default, enabling pure auto-discovery mode.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

agent/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cm-dashboard-agent"
-version = "0.1.134"
+version = "0.1.135"
 edition = "2021"
 
 [dependencies]

agent/src/config/mod.rs

@@ -74,7 +74,8 @@ pub struct DiskConfig {
     pub usage_warning_percent: f32,
     /// Disk usage critical threshold (percentage)
     pub usage_critical_percent: f32,
-    /// Filesystem configurations
+    /// Filesystem configurations (optional - auto-discovery used if empty)
+    #[serde(default)]
     pub filesystems: Vec<FilesystemConfig>,
     /// SMART monitoring thresholds
     pub temperature_warning_celsius: f32,