feat(audit): DegradingSink + durable buffer + loki/ntfy + reconcile (#54)

The classification-aware I5 audit path (§4.4):

- DegradingSink.Reserve: central up → AuditCentral; central down +
  confidential → refuse (no buffer); central down + internal/public +
  buffer writable → AuditBuffered; central down + buffer unwritable →
  floor refuse. Record executes the reserved outcome and, when buffered,
  fires an ntfy alert.
- FileBuffer: durable JSONL buffer that survives process restart; Confirm
  rewrites the file without a record, so a buffered record is cleared ONLY
  after its central write is confirmed.
- LokiCentral: /ready probe + /loki/api/v1/push (full audit entry as the
  structured line). NtfyNotifier: degraded-state alerts; token only in the
  auth header, never logged (regression-tested).
- Reconcile + StartReconcile: replay buffered records to central on
  recovery, confirm-then-clear per record; a failed push keeps the record
  buffered (no loss). SlogSink updated to the two-phase shape (always
  central, never fails) — the default when no loki endpoint is set.

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

ingestion/internal/audit/slog_test.go

@@ -22,7 +22,7 @@ func TestSlogSinkRecordsEntryAndSecurityEvents(t *testing.T) {
 		EffectiveClassification: "confidential",
 		Items:                   []string{"insight:wiki/a/facts/x.md"},
 		SecurityEvents:          []string{"asserted-vs-derived origin mismatch"},
-	})
+	}, capture.AuditCentral)
 	require.NoError(t, err)
 
 	out := buf.String()
@@ -36,6 +36,6 @@ func TestSlogSinkRecordsEntryAndSecurityEvents(t *testing.T) {
 func TestSlogSinkNilLoggerDefaults(t *testing.T) {
 	// nil logger must not panic.
 	require.NotPanics(t, func() {
-		_ = audit.NewSlogSink(nil).Record(context.Background(), capture.AuditEntry{})
+		_ = audit.NewSlogSink(nil).Record(context.Background(), capture.AuditEntry{}, capture.AuditCentral)
 	})
 }