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>

testmain_test.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"io"
+	"os"
+	"testing"
+)
+
+// TestMain silences issue-log output for the entire test binary.
+// Tests still exercise error paths and assert via errorCount; we just
+// don't want the human-facing "[ERROR] ..." prints polluting CI logs
+// (GitHub Actions auto-annotates "[ERROR]" lines as workflow errors).
+func TestMain(m *testing.M) {
+	issueLogWriter = io.Discard
+	os.Exit(m.Run())
+}