feat: add config package with types and JSON serialization

Types:
- TownConfig: main town configuration (config/town.json)
- RigsConfig: rigs registry with BeadsConfig per rig
- AgentState: agent state with role, session, extras

Features:
- Load/Save functions for all types
- Version compatibility checks
- Required field validation
- Creates parent directories on save

Closes gt-f9x.1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/loader.go

@@ -0,0 +1,188 @@
+package config
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+var (
+	// ErrNotFound indicates the config file does not exist.
+	ErrNotFound = errors.New("config file not found")
+
+	// ErrInvalidVersion indicates an unsupported schema version.
+	ErrInvalidVersion = errors.New("unsupported config version")
+
+	// ErrInvalidType indicates an unexpected config type.
+	ErrInvalidType = errors.New("invalid config type")
+
+	// ErrMissingField indicates a required field is missing.
+	ErrMissingField = errors.New("missing required field")
+)
+
+// LoadTownConfig loads and validates a town configuration file.
+func LoadTownConfig(path string) (*TownConfig, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
+		}
+		return nil, fmt.Errorf("reading config: %w", err)
+	}
+
+	var config TownConfig
+	if err := json.Unmarshal(data, &config); err != nil {
+		return nil, fmt.Errorf("parsing config: %w", err)
+	}
+
+	if err := validateTownConfig(&config); err != nil {
+		return nil, err
+	}
+
+	return &config, nil
+}
+
+// SaveTownConfig saves a town configuration to a file.
+func SaveTownConfig(path string, config *TownConfig) error {
+	if err := validateTownConfig(config); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating directory: %w", err)
+	}
+
+	data, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding config: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return fmt.Errorf("writing config: %w", err)
+	}
+
+	return nil
+}
+
+// LoadRigsConfig loads and validates a rigs registry file.
+func LoadRigsConfig(path string) (*RigsConfig, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
+		}
+		return nil, fmt.Errorf("reading config: %w", err)
+	}
+
+	var config RigsConfig
+	if err := json.Unmarshal(data, &config); err != nil {
+		return nil, fmt.Errorf("parsing config: %w", err)
+	}
+
+	if err := validateRigsConfig(&config); err != nil {
+		return nil, err
+	}
+
+	return &config, nil
+}
+
+// SaveRigsConfig saves a rigs registry to a file.
+func SaveRigsConfig(path string, config *RigsConfig) error {
+	if err := validateRigsConfig(config); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating directory: %w", err)
+	}
+
+	data, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding config: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return fmt.Errorf("writing config: %w", err)
+	}
+
+	return nil
+}
+
+// LoadAgentState loads an agent state file.
+func LoadAgentState(path string) (*AgentState, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
+		}
+		return nil, fmt.Errorf("reading state: %w", err)
+	}
+
+	var state AgentState
+	if err := json.Unmarshal(data, &state); err != nil {
+		return nil, fmt.Errorf("parsing state: %w", err)
+	}
+
+	if err := validateAgentState(&state); err != nil {
+		return nil, err
+	}
+
+	return &state, nil
+}
+
+// SaveAgentState saves an agent state to a file.
+func SaveAgentState(path string, state *AgentState) error {
+	if err := validateAgentState(state); err != nil {
+		return err
+	}
+
+	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
+		return fmt.Errorf("creating directory: %w", err)
+	}
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return fmt.Errorf("encoding state: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0644); err != nil {
+		return fmt.Errorf("writing state: %w", err)
+	}
+
+	return nil
+}
+
+// validateTownConfig validates a TownConfig.
+func validateTownConfig(c *TownConfig) error {
+	if c.Type != "town" && c.Type != "" {
+		return fmt.Errorf("%w: expected type 'town', got '%s'", ErrInvalidType, c.Type)
+	}
+	if c.Version > CurrentTownVersion {
+		return fmt.Errorf("%w: got %d, max supported %d", ErrInvalidVersion, c.Version, CurrentTownVersion)
+	}
+	if c.Name == "" {
+		return fmt.Errorf("%w: name", ErrMissingField)
+	}
+	return nil
+}
+
+// validateRigsConfig validates a RigsConfig.
+func validateRigsConfig(c *RigsConfig) error {
+	if c.Version > CurrentRigsVersion {
+		return fmt.Errorf("%w: got %d, max supported %d", ErrInvalidVersion, c.Version, CurrentRigsVersion)
+	}
+	if c.Rigs == nil {
+		c.Rigs = make(map[string]RigEntry)
+	}
+	return nil
+}
+
+// validateAgentState validates an AgentState.
+func validateAgentState(s *AgentState) error {
+	if s.Role == "" {
+		return fmt.Errorf("%w: role", ErrMissingField)
+	}
+	return nil
+}