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.go

@@ -0,0 +1,48 @@
+// Package audit provides AuditSink implementations for the capture
+// capability (I5). This file ships the minimal slog-backed sink used in
+// #53: it emits the request-level audit record to structured logs, which
+// the alloy/loki substrate already scrapes. The classification-aware
+// degradation/refusal sink (confidential fails closed, internal buffers +
+// reconciles) lands in #54 and replaces this behind the same interface.
+package audit
+
+import (
+	"context"
+	"log/slog"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+)
+
+// SlogSink records audit entries to an slog.Logger. It never fails, so it
+// does not exercise the I5 floor (refuse-if-unauditable) — that is #54's
+// loki+buffer sink. A nil logger falls back to slog.Default().
+type SlogSink struct {
+	logger *slog.Logger
+}
+
+// NewSlogSink constructs a SlogSink. nil logger ⇒ slog.Default().
+func NewSlogSink(logger *slog.Logger) *SlogSink {
+	if logger == nil {
+		logger = slog.Default()
+	}
+	return &SlogSink{logger: logger}
+}
+
+// Record emits the audit entry at info level. Security events, when
+// present, are logged at warn level so they surface independently of the
+// routine audit stream.
+func (s *SlogSink) Record(_ context.Context, e capture.AuditEntry) error {
+	s.logger.Info("capture audit",
+		"principal", e.Principal,
+		"actor", e.Actor,
+		"harness", e.Harness,
+		"session_ref", e.SessionRef,
+		"classification", e.EffectiveClassification,
+		"items", e.Items,
+		"ts", e.Timestamp,
+	)
+	for _, ev := range e.SecurityEvents {
+		s.logger.Warn("capture security event", "principal", e.Principal, "event", ev)
+	}
+	return nil
+}