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.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -17,12 +18,15 @@ var (
 	issues     []string
 	notices    []string
 	errorCount int
+	// issueLogWriter is the destination for human-facing issue log lines.
+	// Overridden during tests to suppress intentional error-path output.
+	issueLogWriter io.Writer = os.Stdout
 )
 
 func logIssue(level, msg string) {
 	issuesMu.Lock()
 	defer issuesMu.Unlock()
-	fmt.Printf("  [%s] %s\n", level, msg)
+	fmt.Fprintf(issueLogWriter, "  [%s] %s\n", level, msg)
 	issues = append(issues, fmt.Sprintf("[%s] %s", level, msg))
 	if level == "ERROR" {
 		errorCount++