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_test.go

@@ -0,0 +1,132 @@
+package config
+
+import (
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestTownConfigRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config", "town.json")
+
+	original := &TownConfig{
+		Type:      "town",
+		Version:   1,
+		Name:      "test-town",
+		CreatedAt: time.Now().Truncate(time.Second),
+	}
+
+	if err := SaveTownConfig(path, original); err != nil {
+		t.Fatalf("SaveTownConfig: %v", err)
+	}
+
+	loaded, err := LoadTownConfig(path)
+	if err != nil {
+		t.Fatalf("LoadTownConfig: %v", err)
+	}
+
+	if loaded.Name != original.Name {
+		t.Errorf("Name = %q, want %q", loaded.Name, original.Name)
+	}
+	if loaded.Type != original.Type {
+		t.Errorf("Type = %q, want %q", loaded.Type, original.Type)
+	}
+}
+
+func TestRigsConfigRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config", "rigs.json")
+
+	original := &RigsConfig{
+		Version: 1,
+		Rigs: map[string]RigEntry{
+			"gastown": {
+				GitURL:  "git@github.com:steveyegge/gastown.git",
+				AddedAt: time.Now().Truncate(time.Second),
+				BeadsConfig: &BeadsConfig{
+					Repo:   "local",
+					Prefix: "gt-",
+				},
+			},
+		},
+	}
+
+	if err := SaveRigsConfig(path, original); err != nil {
+		t.Fatalf("SaveRigsConfig: %v", err)
+	}
+
+	loaded, err := LoadRigsConfig(path)
+	if err != nil {
+		t.Fatalf("LoadRigsConfig: %v", err)
+	}
+
+	if len(loaded.Rigs) != 1 {
+		t.Errorf("Rigs count = %d, want 1", len(loaded.Rigs))
+	}
+
+	rig, ok := loaded.Rigs["gastown"]
+	if !ok {
+		t.Fatal("missing 'gastown' rig")
+	}
+	if rig.BeadsConfig == nil || rig.BeadsConfig.Prefix != "gt-" {
+		t.Errorf("BeadsConfig.Prefix = %v, want 'gt-'", rig.BeadsConfig)
+	}
+}
+
+func TestAgentStateRoundTrip(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "state.json")
+
+	original := &AgentState{
+		Role:       "mayor",
+		LastActive: time.Now().Truncate(time.Second),
+		Session:    "abc123",
+		Extra: map[string]any{
+			"custom": "value",
+		},
+	}
+
+	if err := SaveAgentState(path, original); err != nil {
+		t.Fatalf("SaveAgentState: %v", err)
+	}
+
+	loaded, err := LoadAgentState(path)
+	if err != nil {
+		t.Fatalf("LoadAgentState: %v", err)
+	}
+
+	if loaded.Role != original.Role {
+		t.Errorf("Role = %q, want %q", loaded.Role, original.Role)
+	}
+	if loaded.Session != original.Session {
+		t.Errorf("Session = %q, want %q", loaded.Session, original.Session)
+	}
+}
+
+func TestLoadTownConfigNotFound(t *testing.T) {
+	_, err := LoadTownConfig("/nonexistent/path.json")
+	if err == nil {
+		t.Fatal("expected error for nonexistent file")
+	}
+}
+
+func TestValidationErrors(t *testing.T) {
+	// Missing name
+	tc := &TownConfig{Type: "town", Version: 1}
+	if err := validateTownConfig(tc); err == nil {
+		t.Error("expected error for missing name")
+	}
+
+	// Wrong type
+	tc = &TownConfig{Type: "wrong", Version: 1, Name: "test"}
+	if err := validateTownConfig(tc); err == nil {
+		t.Error("expected error for wrong type")
+	}
+
+	// Missing role
+	as := &AgentState{}
+	if err := validateAgentState(as); err == nil {
+		t.Error("expected error for missing role")
+	}
+}