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/ntfy.go

@@ -0,0 +1,55 @@
+package audit
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// NtfyNotifier posts alerts to an ntfy topic URL. Used to surface a
+// degraded audit state (records buffered locally during a loki outage).
+type NtfyNotifier struct {
+	topicURL string
+	token    string
+	http     *http.Client
+}
+
+// NewNtfyNotifier constructs a notifier for the given ntfy topic URL
+// (e.g. https://ntfy.sh/my-topic). token is an optional bearer for
+// protected ntfy instances; it is held here and only sent in the
+// Authorization header, never logged. Returns nil when topicURL is empty.
+func NewNtfyNotifier(topicURL, token string) *NtfyNotifier {
+	if topicURL == "" {
+		return nil
+	}
+	return &NtfyNotifier{
+		topicURL: strings.TrimRight(topicURL, "/"),
+		token:    token,
+		http:     &http.Client{Timeout: 10 * time.Second},
+	}
+}
+
+// Notify posts a message to the ntfy topic.
+func (n *NtfyNotifier) Notify(ctx context.Context, msg string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, n.topicURL, strings.NewReader(msg))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("Title", "brain-capture audit degraded")
+	req.Header.Set("Priority", "high")
+	req.Header.Set("Tags", "warning,brain")
+	if n.token != "" {
+		req.Header.Set("Authorization", "Bearer "+n.token)
+	}
+	resp, err := n.http.Do(req)
+	if err != nil {
+		return fmt.Errorf("ntfy notify: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		return fmt.Errorf("ntfy notify: status %d", resp.StatusCode)
+	}
+	return nil
+}