fix: handle empty USERNAME and add debug log for icacls success

- Check for empty USERNAME env var before running icacls to avoid a
  doomed invocation with ":F" grant argument
- Log a clear warning when USERNAME is empty
- Add tracing::debug on successful permission set

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

src/security/secrets.rs

@@ -191,13 +191,17 @@ impl SecretStore {
             #[cfg(windows)]
             {
                 // On Windows, use icacls to restrict permissions to current user only
+                let username = std::env::var("USERNAME").unwrap_or_default();
+                if username.is_empty() {
+                    tracing::warn!(
+                        "USERNAME environment variable is empty; \
+                         cannot restrict key file permissions via icacls"
+                    );
+                } else {
                     match std::process::Command::new("icacls")
                         .arg(&self.key_path)
                         .args(["/inheritance:r", "/grant:r"])
-                    .arg(format!(
-                        "{}:F",
-                        std::env::var("USERNAME").unwrap_or_default()
-                    ))
+                        .arg(format!("{username}:F"))
                         .output()
                     {
                         Ok(o) if !o.status.success() => {
@@ -209,7 +213,10 @@ impl SecretStore {
                         Err(e) => {
                             tracing::warn!("Could not set key file permissions: {e}");
                         }
-                    _ => {}
+                        _ => {
+                            tracing::debug!("Key file permissions restricted via icacls");
+                        }
+                    }
                 }
             }