hive-agent: don't claim 'already logged out' when a credential file is stuck undeletable

Closes #2831. post_logout's wipe_summary only checked whether cleared.wiped
was empty, so a real .credentials.json blocked by EPERM (nothing made it
into wiped) got reported as 'no credential files present (already logged
out)' in the same breath as a warning saying the delete failed -- self
contradictory and misleading, since the file is still there un-deleted.

Extracted the branch into a pure wipe_summary() helper (wiped file(s) named >
warnings-only failure > genuinely nothing found) with unit tests covering
the exact permission-error shape from the issue.

hive-agent/src/web_ui/auth.rs

@@ -89,11 +89,7 @@ pub(super) async fn post_logout(State(state): State<AppState>) -> Response {
     // the file set and preserves session-history files alongside them.
     let dir = crate::paths::claude_dir();
     let cleared = crate::login::clear_session(&dir).await;
-    let wipe_summary = if cleared.wiped.is_empty() {
-        "no credential files present (already logged out)".to_owned()
-    } else {
-        format!("wiped {}", cleared.wiped.join(", "))
-    };
+    let wipe_summary = wipe_summary(&cleared);
     let warn_suffix = if cleared.warnings.is_empty() {
         String::new()
     } else {
@@ -115,3 +111,68 @@ pub(super) async fn post_logout(State(state): State<AppState>) -> Response {
     )
         .into_response()
 }
+
+/// Human-readable summary of a [`crate::login::ClearedSession`] outcome for
+/// the `/api/logout` response + note. Deliberately distinguishes "nothing to
+/// delete" from "something's there but we couldn't delete it" — both leave
+/// `wiped` empty, but only the former is actually "already logged out".
+/// These two used to be conflated, so a stuck undeletable `.credentials.json`
+/// got reported as "already logged out" in the same breath as a warning
+/// saying the delete failed.
+fn wipe_summary(cleared: &crate::login::ClearedSession) -> String {
+    if !cleared.wiped.is_empty() {
+        format!("wiped {}", cleared.wiped.join(", "))
+    } else if cleared.warnings.is_empty() {
+        "no credential files present (already logged out)".to_owned()
+    } else {
+        "failed to delete existing credential file(s), NOT logged out".to_owned()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::wipe_summary;
+    use crate::login::ClearedSession;
+
+    #[test]
+    fn nothing_found_reports_already_logged_out() {
+        let cleared = ClearedSession::default();
+        assert_eq!(
+            wipe_summary(&cleared),
+            "no credential files present (already logged out)"
+        );
+    }
+
+    #[test]
+    fn wiped_files_are_named() {
+        let cleared = ClearedSession {
+            wiped: vec![".credentials.json"],
+            warnings: vec![],
+        };
+        assert_eq!(wipe_summary(&cleared), "wiped .credentials.json");
+    }
+
+    #[test]
+    fn permission_error_does_not_claim_already_logged_out() {
+        // A real file blocked by EPERM must not be reported the same way
+        // as a genuinely absent one.
+        let cleared = ClearedSession {
+            wiped: vec![],
+            warnings: vec![".credentials.json: Permission denied (os error 13)".to_owned()],
+        };
+        let summary = wipe_summary(&cleared);
+        assert!(!summary.contains("already logged out"));
+        assert!(summary.contains("failed to delete"));
+    }
+
+    #[test]
+    fn partial_wipe_with_warnings_still_reports_wiped_files() {
+        // A mix of successes and failures should favor telling the operator
+        // what actually happened over the negative "nothing" framing.
+        let cleared = ClearedSession {
+            wiped: vec![".credentials.json"],
+            warnings: vec!["settings.json: Permission denied (os error 13)".to_owned()],
+        };
+        assert_eq!(wipe_summary(&cleared), "wiped .credentials.json");
+    }
+}