feat(audit): minimal slog AuditSink for capture I5 (#53)

Emits the request-level audit record to structured logs (scraped by the
existing alloy/loki substrate) and surfaces security events at warn
level. Placeholder behind the AuditSink interface — the classification-
aware loki+buffer+reconcile sink (confidential fails closed, internal
degrades) lands in #54 and replaces this without touching callers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/audit/slog_test.go

@@ -0,0 +1,41 @@
+package audit_test
+
+import (
+	"bytes"
+	"context"
+	"log/slog"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/audit"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSlogSinkRecordsEntryAndSecurityEvents(t *testing.T) {
+	var buf bytes.Buffer
+	sink := audit.NewSlogSink(slog.New(slog.NewTextHandler(&buf, nil)))
+
+	err := sink.Record(context.Background(), capture.AuditEntry{
+		Principal:               "koala-cli",
+		Harness:                 "claude-code",
+		EffectiveClassification: "confidential",
+		Items:                   []string{"insight:wiki/a/facts/x.md"},
+		SecurityEvents:          []string{"asserted-vs-derived origin mismatch"},
+	})
+	require.NoError(t, err)
+
+	out := buf.String()
+	assert.Contains(t, out, "capture audit")
+	assert.Contains(t, out, "koala-cli")
+	assert.Contains(t, out, "confidential")
+	assert.Contains(t, out, "capture security event")
+	assert.Contains(t, out, "asserted-vs-derived origin mismatch")
+}
+
+func TestSlogSinkNilLoggerDefaults(t *testing.T) {
+	// nil logger must not panic.
+	require.NotPanics(t, func() {
+		_ = audit.NewSlogSink(nil).Record(context.Background(), capture.AuditEntry{})
+	})
+}