Annotate failures for Github Actions so they're easier to spot.

Tools/Sources/Commands/CI/CI.swift

@@ -45,11 +45,21 @@ struct CI: ParsableCommand {
             try await run(.name("swiftformat"), ["--lint", "."])
         } catch {
             logger.error("\n❌ SwiftFormat failed.\n")
+            annotateError(title: "SwiftFormat lint failed", "Run `swiftformat .` from the project root and commit the changes.")
             throw error
         }
         logger.info("\n✅ SwiftFormat passed.\n")
     }
     
+    // MARK: - GitHub Actions
+    
+    /// Logs an error annotation, surfacing the failure on the workflow run's summary page.
+    ///
+    /// This is printed directly as the workflow command must be at the very start of the line.
+    static func annotateError(title: String, _ message: String) {
+        print("::error title=\(title)::\(message)")
+    }
+    
     // MARK: - Test Results
     
     /// Collects coverage from an xcresult bundle using xcresultparser (cobertura format).