perf(codex): fall back on incremental token error; slice before file extract

Address review feedback:
- computeIncrementalTokenUsage now falls back to the full-scan token calc on
  incremental error instead of silently dropping the checkpoint's token usage.
- ExtractAllModifiedFiles slices to the post-offset lines before splitting, so
  per-turn work scales with the delta rather than the whole session.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Entire-Checkpoint: 01KY9HW4BJJRTQ7T5TMK7KGPR6

Author: gtrrz-victor

cmd/entire/cli/agent/codex/transcript.go

@@ -386,14 +386,15 @@ func scanTokenDelta(data []byte, startLineNum, fromOffset int, seed *tokenUsageD
 // transcript bytes it already holds, instead of falling back to a second full-file
 // disk read via ExtractModifiedFilesFromOffset.
 func (c *CodexAgent) ExtractAllModifiedFiles(transcriptData []byte, fromOffset int, _ string) ([]string, error) {
+	// Slice to the lines added since fromOffset before splitting, so per-turn work
+	// scales with the delta rather than the whole session.
+	data := transcriptData
+	if fromOffset > 0 {
+		data = transcript.SliceFromLine(transcriptData, fromOffset)
+	}
 	seen := make(map[string]struct{})
 	var files []string
-	lineNum := 0
-	for _, lineData := range splitJSONL(transcriptData) {
-		lineNum++
-		if lineNum <= fromOffset {
-			continue
-		}
+	for _, lineData := range splitJSONL(data) {
 		for _, f := range extractFilesFromLine(lineData) {
 			if _, ok := seen[f]; !ok {
 				seen[f] = struct{}{}

cmd/entire/cli/lifecycle.go

@@ -1011,9 +1011,18 @@ func computeIncrementalTokenUsage(ctx, logCtx context.Context, inc agent.Increme
 	}
 	usage, next, err := inc.CalculateTokenUsageIncremental(transcriptData, fromOffset, prior)
 	if err != nil {
-		logging.Debug(logCtx, "incremental token calculation failed",
+		// Fall back to the full-scan path so a checkpoint's token usage is not
+		// silently dropped; skip persisting a baseline so the next hook re-derives
+		// one from a clean full scan.
+		logging.Debug(logCtx, "incremental token calculation failed, falling back to full scan",
 			slog.String("error", err.Error()))
-		return nil, nil
+		fallback, fbErr := inc.CalculateTokenUsage(transcriptData, fromOffset)
+		if fbErr != nil {
+			logging.Debug(logCtx, "full-scan token calculation also failed",
+				slog.String("error", fbErr.Error()))
+			return nil, nil
+		}
+		return fallback, nil
 	}
 	return usage, next
 }