Extract safecsv writer for spreadsheet-safe exports

Wrap encoding/csv with formula-safe cell sanitization and
use it from log export streaming.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/safecsv/cell_test.go

@@ -0,0 +1,52 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package safecsv
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSanitizeCell(t *testing.T) {
+	t.Parallel()
+
+	assert.Equal(t, "plain", SanitizeCell("plain"))
+	assert.Equal(t, "'=1+1", SanitizeCell("=1+1"))
+	assert.Equal(t, "'+cmd", SanitizeCell("+cmd"))
+	assert.Equal(t, "'-2", SanitizeCell("-2"))
+	assert.Equal(t, "'@sum", SanitizeCell("@sum"))
+}
+
+func TestWriterWrite(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	w := NewWriter(&buf)
+	require.NoError(t, w.WriteRow("ok", "=evil"))
+	w.Flush()
+	require.NoError(t, w.Error())
+
+	assert.Equal(t, "ok,'=evil\n", buf.String())
+}