Suppress intentional errLog output during tests

Tests that exercise error paths in errLog/warn were causing
GitHub Actions to annotate test output with 'Error: R] ...'
(GHA eats '[ERRO' from '[ERROR]' lines and surfaces the rest as
workflow errors), making real failures hard to distinguish from
expected behavior.

- issues.go: route logIssue output through issueLogWriter (io.Writer,
  defaults to os.Stdout). errorCount/issues/notices recording is
  unchanged so tests still assert via errorCount.
- testmain_test.go: TestMain sets issueLogWriter = io.Discard for
  the whole test binary.
- issues_test.go: TestIssuesLogging temporarily redirects
  issueLogWriter to a bytes.Buffer (replacing os.Pipe stdout
  capture).

go test -v ./... now contains zero stray '[ERROR]' lines; real
test failures remain visible and 'go test' still exits non-zero
on failure, failing the workflow.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

issues_test.go

@@ -12,27 +12,23 @@ func TestIssuesLogging(t *testing.T) {
 	defer resetMocks()
 	resetMocks()
 
-	// Capture stdout
-	oldStdout := os.Stdout
-	r, w, _ := os.Pipe()
-	os.Stdout = w
+	// Capture issue log output via the injectable writer.
+	var buf bytes.Buffer
+	oldWriter := issueLogWriter
+	issueLogWriter = &buf
+	defer func() { issueLogWriter = oldWriter }()
 
 	warn("something is deprecated")
 	errLog("something failed")
 	notice("please restart shell")
 
-	w.Close()
-	os.Stdout = oldStdout
-
-	var buf bytes.Buffer
-	io.Copy(&buf, r)
 	output := buf.String()
 
 	if !strings.Contains(output, "[WARN] something is deprecated") {
-		t.Errorf("stdout missing warning: %q", output)
+		t.Errorf("output missing warning: %q", output)
 	}
 	if !strings.Contains(output, "[ERROR] something failed") {
-		t.Errorf("stdout missing error: %q", output)
+		t.Errorf("output missing error: %q", output)
 	}
 
 	issuesMu.Lock()