Add config.json support for database path configuration (bd-163)

- Create internal/configfile package for config.json handling
- bd init now creates .beads/config.json with database, version, and jsonl_export fields
- Database discovery checks config.json first, falls back to beads.db
- Update .gitignore to not ignore config.json (part of repo state)
- Update test to expect beads.db and config.json
- Backward compatible with existing beads.db-only setups

internal/configfile/configfile_test.go

@@ -0,0 +1,128 @@
+package configfile
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDefaultConfig(t *testing.T) {
+	cfg := DefaultConfig("0.17.5")
+	
+	if cfg.Database != "beads.db" {
+		t.Errorf("Database = %q, want beads.db", cfg.Database)
+	}
+	
+	if cfg.Version != "0.17.5" {
+		t.Errorf("Version = %q, want 0.17.5", cfg.Version)
+	}
+	
+	if cfg.JSONLExport != "beads.jsonl" {
+		t.Errorf("JSONLExport = %q, want beads.jsonl", cfg.JSONLExport)
+	}
+}
+
+func TestLoadSaveRoundtrip(t *testing.T) {
+	tmpDir := t.TempDir()
+	beadsDir := filepath.Join(tmpDir, ".beads")
+	if err := os.MkdirAll(beadsDir, 0750); err != nil {
+		t.Fatalf("failed to create .beads directory: %v", err)
+	}
+	
+	cfg := DefaultConfig("0.17.5")
+	
+	if err := cfg.Save(beadsDir); err != nil {
+		t.Fatalf("Save() failed: %v", err)
+	}
+	
+	loaded, err := Load(beadsDir)
+	if err != nil {
+		t.Fatalf("Load() failed: %v", err)
+	}
+	
+	if loaded == nil {
+		t.Fatal("Load() returned nil config")
+	}
+	
+	if loaded.Database != cfg.Database {
+		t.Errorf("Database = %q, want %q", loaded.Database, cfg.Database)
+	}
+	
+	if loaded.Version != cfg.Version {
+		t.Errorf("Version = %q, want %q", loaded.Version, cfg.Version)
+	}
+	
+	if loaded.JSONLExport != cfg.JSONLExport {
+		t.Errorf("JSONLExport = %q, want %q", loaded.JSONLExport, cfg.JSONLExport)
+	}
+}
+
+func TestLoadNonexistent(t *testing.T) {
+	tmpDir := t.TempDir()
+	
+	cfg, err := Load(tmpDir)
+	if err != nil {
+		t.Fatalf("Load() returned error for nonexistent config: %v", err)
+	}
+	
+	if cfg != nil {
+		t.Errorf("Load() = %v, want nil for nonexistent config", cfg)
+	}
+}
+
+func TestDatabasePath(t *testing.T) {
+	beadsDir := "/home/user/project/.beads"
+	cfg := &Config{Database: "beads.db"}
+	
+	got := cfg.DatabasePath(beadsDir)
+	want := filepath.Join(beadsDir, "beads.db")
+	
+	if got != want {
+		t.Errorf("DatabasePath() = %q, want %q", got, want)
+	}
+}
+
+func TestJSONLPath(t *testing.T) {
+	beadsDir := "/home/user/project/.beads"
+	
+	tests := []struct {
+		name       string
+		cfg        *Config
+		want       string
+	}{
+		{
+			name: "default",
+			cfg:  &Config{JSONLExport: "beads.jsonl"},
+			want: filepath.Join(beadsDir, "beads.jsonl"),
+		},
+		{
+			name: "custom",
+			cfg:  &Config{JSONLExport: "custom.jsonl"},
+			want: filepath.Join(beadsDir, "custom.jsonl"),
+		},
+		{
+			name: "empty falls back to default",
+			cfg:  &Config{JSONLExport: ""},
+			want: filepath.Join(beadsDir, "beads.jsonl"),
+		},
+	}
+	
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.cfg.JSONLPath(beadsDir)
+			if got != tt.want {
+				t.Errorf("JSONLPath() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestConfigPath(t *testing.T) {
+	beadsDir := "/home/user/project/.beads"
+	got := ConfigPath(beadsDir)
+	want := filepath.Join(beadsDir, "config.json")
+	
+	if got != want {
+		t.Errorf("ConfigPath() = %q, want %q", got, want)
+	}
+}