fix(codebuff): address review findings - timestamps, freshness

1. Usage timestamp fallback: When no message timestamp is parsable,
   fall back to session directory timestamp, then source mtime, instead
   of producing year-0001 timestamps.

2. Source freshness: Added Codebuff-specific SourceMtime that computes
   composite max mtime across chat-messages.json, run-state.json, and
   chat-meta.json so companion-file-only changes are caught.

Author: scross01

internal/parser/codebuff.go

@@ -197,19 +197,26 @@ func parseCodebuffSession(
 	// empty. Cost is tracked at the session level via credits.
 	if rs.CreditsUsed > 0 {
 		cost := rs.CreditsUsed * 0.01
+		// Determine occurred_at: prefer message timestamps, then fall
+		// back to the session directory timestamp, then source mtime.
+		occurredAt := startedAt
+		if !endedAt.IsZero() {
+			occurredAt = endedAt
+		}
+		if occurredAt.IsZero() && !sessionDate.IsZero() {
+			occurredAt = sessionDate
+		}
+		if occurredAt.IsZero() && fileInfo.Mtime > 0 {
+			occurredAt = time.Unix(0, fileInfo.Mtime)
+		}
 		sess.UsageEvents = []ParsedUsageEvent{{
-			SessionID: fullID,
-			Source:    "session",
-			OccurredAt: func() string {
-				if !endedAt.IsZero() {
-					return endedAt.Format(time.RFC3339Nano)
-				}
-				return startedAt.Format(time.RFC3339Nano)
-			}(),
-			CostUSD:    &cost,
-			CostStatus: "reported",
-			CostSource: "session",
-			DedupKey:   "session:" + fullID,
+			SessionID:   fullID,
+			Source:      "session",
+			OccurredAt:  occurredAt.Format(time.RFC3339Nano),
+			CostUSD:     &cost,
+			CostStatus:  "reported",
+			CostSource:  "session",
+			DedupKey:    "session:" + fullID,
 		}}
 	}
 

internal/sync/engine.go

@@ -12156,6 +12156,26 @@ func (e *Engine) SourceMtime(sessionID string) int64 {
 		_, mtime := roocodeEffectiveStat(path, info)
 		return mtime
 	}
+	if def.Type == parser.AgentCodebuff {
+		// Freshness spans chat-messages.json plus run-state.json and
+		// chat-meta.json. Compute the composite max mtime across all
+		// three files so companion-file-only changes are caught.
+		info, err := os.Stat(path)
+		if err != nil {
+			return 0
+		}
+		mtime := info.ModTime().UnixNano()
+		dir := filepath.Dir(path)
+		for _, name := range []string{"run-state.json", "chat-meta.json"} {
+			companion := filepath.Join(dir, name)
+			if ci, err := os.Stat(companion); err == nil {
+				if cm := ci.ModTime().UnixNano(); cm > mtime {
+					mtime = cm
+				}
+			}
+		}
+		return mtime
+	}
 	if def.Type == parser.AgentKiloLegacy {
 		// Freshness spans task_metadata.json (the stored path) plus
 		// its siblings ui_messages.json and api_conversation_history.json.