chore(clippy): clear warning backlog and harden conversions (#383)

src/config/schema.rs

@@ -1199,7 +1199,7 @@ pub struct LarkConfig {
 // ── Security Config ─────────────────────────────────────────────────
 
 /// Security configuration for sandboxing, resource limits, and audit logging
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct SecurityConfig {
     /// Sandbox configuration
     #[serde(default)]
@@ -1214,16 +1214,6 @@ pub struct SecurityConfig {
     pub audit: AuditConfig,
 }
 
-impl Default for SecurityConfig {
-    fn default() -> Self {
-        Self {
-            sandbox: SandboxConfig::default(),
-            resources: ResourceLimitsConfig::default(),
-            audit: AuditConfig::default(),
-        }
-    }
-}
-
 /// Sandbox configuration for OS-level isolation
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct SandboxConfig {
@@ -1251,10 +1241,11 @@ impl Default for SandboxConfig {
 }
 
 /// Sandbox backend selection
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, Default)]
 #[serde(rename_all = "lowercase")]
 pub enum SandboxBackend {
     /// Auto-detect best available (default)
+    #[default]
     Auto,
     /// Landlock (Linux kernel LSM, native)
     Landlock,
@@ -1268,12 +1259,6 @@ pub enum SandboxBackend {
     None,
 }
 
-impl Default for SandboxBackend {
-    fn default() -> Self {
-        Self::Auto
-    }
-}
-
 /// Resource limits for command execution
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ResourceLimitsConfig {