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/buffer.go

@@ -0,0 +1,167 @@
+package audit
+
+import (
+	"bufio"
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/capture"
+)
+
+// FileBuffer is a durable, restart-surviving audit buffer backed by a
+// JSONL file: one {id, entry} record per line. It is the internal/public
+// tier fallback when loki is unreachable. Confirm rewrites the file
+// without the confirmed record, so a record is cleared only after its
+// central write is confirmed.
+//
+// Access is serialised by a mutex; the buffer is low-throughput (only
+// written during a loki outage), so a whole-file rewrite on Confirm is
+// acceptable and keeps the on-disk format trivially correct.
+type FileBuffer struct {
+	path string
+	mu   sync.Mutex
+}
+
+type bufferLine struct {
+	ID    string              `json:"id"`
+	Entry capture.AuditEntry  `json:"entry"`
+}
+
+// NewFileBuffer returns a buffer backed by path. The parent directory is
+// created if needed. The file itself is created lazily on first Append.
+func NewFileBuffer(path string) (*FileBuffer, error) {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return nil, fmt.Errorf("create buffer dir: %w", err)
+	}
+	return &FileBuffer{path: path}, nil
+}
+
+// Writable reports whether the buffer file can be appended to. It probes
+// by opening the file for append (creating it if absent) — the same
+// operation Append performs — so Reserve's check matches Append's reality.
+func (b *FileBuffer) Writable() error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	f, err := os.OpenFile(b.path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return err
+	}
+	return f.Close()
+}
+
+// Append durably writes one audit record. The ID is derived from the
+// content + timestamp so it is stable and unique per record.
+func (b *FileBuffer) Append(e capture.AuditEntry) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	line := bufferLine{ID: recordID(e), Entry: e}
+	data, err := json.Marshal(line)
+	if err != nil {
+		return fmt.Errorf("marshal buffer line: %w", err)
+	}
+	f, err := os.OpenFile(b.path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o644)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = f.Close() }()
+	if _, err := f.Write(append(data, '\n')); err != nil {
+		return err
+	}
+	return f.Sync()
+}
+
+// Pending reads all buffered records. A missing file means none.
+func (b *FileBuffer) Pending() ([]Buffered, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.readAllLocked()
+}
+
+func (b *FileBuffer) readAllLocked() ([]Buffered, error) {
+	f, err := os.Open(b.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	defer func() { _ = f.Close() }()
+
+	var out []Buffered
+	sc := bufio.NewScanner(f)
+	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	for sc.Scan() {
+		raw := sc.Bytes()
+		if len(raw) == 0 {
+			continue
+		}
+		var l bufferLine
+		if err := json.Unmarshal(raw, &l); err != nil {
+			return nil, fmt.Errorf("parse buffer line: %w", err)
+		}
+		out = append(out, Buffered(l))
+	}
+	return out, sc.Err()
+}
+
+// Confirm removes a single record after its central write is confirmed, by
+// rewriting the file without it. Unknown IDs are a no-op.
+func (b *FileBuffer) Confirm(id string) error {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	all, err := b.readAllLocked()
+	if err != nil {
+		return err
+	}
+	tmp := b.path + ".tmp"
+	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0o644)
+	if err != nil {
+		return err
+	}
+	w := bufio.NewWriter(f)
+	kept := 0
+	for _, rec := range all {
+		if rec.ID == id {
+			continue
+		}
+		data, _ := json.Marshal(bufferLine(rec))
+		if _, err := w.Write(append(data, '\n')); err != nil {
+			_ = f.Close()
+			return err
+		}
+		kept++
+	}
+	if err := w.Flush(); err != nil {
+		_ = f.Close()
+		return err
+	}
+	if err := f.Sync(); err != nil {
+		_ = f.Close()
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	// Empty buffer → remove the file entirely so Pending sees nothing.
+	if kept == 0 {
+		_ = os.Remove(tmp)
+		return os.Remove(b.path)
+	}
+	return os.Rename(tmp, b.path)
+}
+
+// recordID is a stable per-record identifier: sha256 of the principal,
+// timestamp, and item list. Distinct captures never collide; the same
+// buffered record always hashes the same.
+func recordID(e capture.AuditEntry) string {
+	h := sha256.New()
+	_, _ = fmt.Fprintf(h, "%s|%s|%v|%s", e.Principal, e.Timestamp.UTC().Format("2006-01-02T15:04:05.000000000Z07:00"), e.Items, e.SessionRef)
+	return hex.EncodeToString(h.Sum(nil))[:16]
+}