feat(deletions): auto-compact during sync and git history fallback fixes

- Add Count function to deletions package for fast line counting
- Add maybeAutoCompactDeletions to sync (opt-in via deletions.auto_compact config)
- Fix regex escaping in batchCheckGitHistory (bd-bgs)
- Add 30s timeout to git history commands (bd-f0n)
- Use git rev-parse --show-toplevel for proper repo root detection (bd-bhd)
- Add tests for Count and auto-compact functionality

Closes: bd-qsm, bd-bgs, bd-f0n, bd-bhd

internal/deletions/deletions.go

@@ -180,6 +180,35 @@ func DefaultPath(beadsDir string) string {
 	return filepath.Join(beadsDir, "deletions.jsonl")
 }
 
+// Count returns the number of lines in the deletions manifest.
+// This is a fast operation that doesn't parse JSON, just counts lines.
+// Returns 0 if the file doesn't exist or is empty.
+func Count(path string) (int, error) {
+	f, err := os.Open(path) // #nosec G304 - controlled path from caller
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 0, nil
+		}
+		return 0, fmt.Errorf("failed to open deletions file: %w", err)
+	}
+	defer f.Close()
+
+	count := 0
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if line != "" {
+			count++
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return 0, fmt.Errorf("error reading deletions file: %w", err)
+	}
+
+	return count, nil
+}
+
 // DefaultRetentionDays is the default number of days to retain deletion records.
 const DefaultRetentionDays = 7
 

internal/deletions/deletions_test.go

@@ -546,3 +546,64 @@ func TestPruneDeletions_ZeroRetention(t *testing.T) {
 		t.Errorf("expected 1 pruned with 0 retention, got %d", result.PrunedCount)
 	}
 }
+
+func TestCount_Empty(t *testing.T) {
+	// Non-existent file should return 0
+	count, err := Count("/nonexistent/path/deletions.jsonl")
+	if err != nil {
+		t.Fatalf("expected no error for non-existent file, got: %v", err)
+	}
+	if count != 0 {
+		t.Errorf("expected 0 count for non-existent file, got %d", count)
+	}
+}
+
+func TestCount_WithRecords(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := filepath.Join(tmpDir, "deletions.jsonl")
+
+	now := time.Now()
+	records := []DeletionRecord{
+		{ID: "bd-001", Timestamp: now, Actor: "user1"},
+		{ID: "bd-002", Timestamp: now, Actor: "user2"},
+		{ID: "bd-003", Timestamp: now, Actor: "user3"},
+	}
+
+	for _, r := range records {
+		if err := AppendDeletion(path, r); err != nil {
+			t.Fatalf("AppendDeletion failed: %v", err)
+		}
+	}
+
+	count, err := Count(path)
+	if err != nil {
+		t.Fatalf("Count failed: %v", err)
+	}
+	if count != 3 {
+		t.Errorf("expected 3, got %d", count)
+	}
+}
+
+func TestCount_WithEmptyLines(t *testing.T) {
+	tmpDir := t.TempDir()
+	path := filepath.Join(tmpDir, "deletions.jsonl")
+
+	// Write content with empty lines
+	content := `{"id":"bd-001","ts":"2024-01-01T00:00:00Z","by":"user1"}
+
+{"id":"bd-002","ts":"2024-01-02T00:00:00Z","by":"user2"}
+
+`
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatalf("failed to write test file: %v", err)
+	}
+
+	count, err := Count(path)
+	if err != nil {
+		t.Fatalf("Count failed: %v", err)
+	}
+	// Should count only non-empty lines
+	if count != 2 {
+		t.Errorf("expected 2 (excluding empty lines), got %d", count)
+	}
+}