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.go

@@ -0,0 +1,73 @@
+package configfile
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+const ConfigFileName = "config.json"
+
+type Config struct {
+	Database     string `json:"database"`
+	Version      string `json:"version"`
+	JSONLExport  string `json:"jsonl_export,omitempty"`
+}
+
+func DefaultConfig(version string) *Config {
+	return &Config{
+		Database:    "beads.db",
+		Version:     version,
+		JSONLExport: "beads.jsonl",
+	}
+}
+
+func ConfigPath(beadsDir string) string {
+	return filepath.Join(beadsDir, ConfigFileName)
+}
+
+func Load(beadsDir string) (*Config, error) {
+	configPath := ConfigPath(beadsDir)
+	
+	data, err := os.ReadFile(configPath)
+	if os.IsNotExist(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("reading config: %w", err)
+	}
+	
+	var cfg Config
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing config: %w", err)
+	}
+	
+	return &cfg, nil
+}
+
+func (c *Config) Save(beadsDir string) error {
+	configPath := ConfigPath(beadsDir)
+	
+	data, err := json.MarshalIndent(c, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshaling config: %w", err)
+	}
+	
+	if err := os.WriteFile(configPath, data, 0644); err != nil {
+		return fmt.Errorf("writing config: %w", err)
+	}
+	
+	return nil
+}
+
+func (c *Config) DatabasePath(beadsDir string) string {
+	return filepath.Join(beadsDir, c.Database)
+}
+
+func (c *Config) JSONLPath(beadsDir string) string {
+	if c.JSONLExport == "" {
+		return filepath.Join(beadsDir, "beads.jsonl")
+	}
+	return filepath.Join(beadsDir, c.JSONLExport)
+}