feat(config): register all defaults in code, shrink example config

Only the database password is truly mandatory to get started, but
config.example.json was 267 lines with 90+ options. Newcomers faced a
wall of settings with no indication of what matters.

Add registerDefaults() with all sane defaults via Viper so a minimal
config (just DB credentials) produces a fully working server. Gameplay
multipliers default to 1.0 instead of Go's zero value 0.0, which
previously zeroed out all quest rewards for minimal configs. Uses
dot-notation defaults for GameplayOptions/DebugOptions so users can
override individual fields without losing other defaults.

- Shrink config.example.json from 267 to 10 lines
- Rename full original to config.reference.json as documentation
- Simplify wizard buildDefaultConfig() from ~220 to ~12 lines
- Fix latent bug: SaveDumps default used wrong key DevModeOptions
- Add tests: minimal config, backward compat, single-field override
- Update release workflow, README, CONTRIBUTING, docker/README

config/config_load_test.go

@@ -2,8 +2,11 @@ package config
 
 import (
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
+
+	"github.com/spf13/viper"
 )
 
 // TestLoadConfigNoFile tests LoadConfig when config file doesn't exist
@@ -497,3 +500,191 @@ func BenchmarkConfigCreation(b *testing.B) {
 		}
 	}
 }
+
+// writeMinimalConfig writes a minimal config.json to dir and returns its path.
+func writeMinimalConfig(t *testing.T, dir, content string) {
+	t.Helper()
+	if err := os.WriteFile(filepath.Join(dir, "config.json"), []byte(content), 0644); err != nil {
+		t.Fatalf("writing config.json: %v", err)
+	}
+}
+
+// TestMinimalConfigDefaults verifies that a minimal config.json produces a fully
+// populated Config with sane defaults (multipliers not zero, entrance entries present, etc).
+func TestMinimalConfigDefaults(t *testing.T) {
+	viper.Reset()
+	dir := t.TempDir()
+	origDir, _ := os.Getwd()
+	defer func() { _ = os.Chdir(origDir) }()
+	if err := os.Chdir(dir); err != nil {
+		t.Fatal(err)
+	}
+
+	writeMinimalConfig(t, dir, `{
+		"Database": { "Password": "test" }
+	}`)
+
+	cfg, err := LoadConfig()
+	if err != nil {
+		t.Fatalf("LoadConfig() error: %v", err)
+	}
+
+	// Multipliers must be 1.0 (not Go's zero value 0.0)
+	multipliers := map[string]float32{
+		"HRPMultiplier":       cfg.GameplayOptions.HRPMultiplier,
+		"SRPMultiplier":       cfg.GameplayOptions.SRPMultiplier,
+		"GRPMultiplier":       cfg.GameplayOptions.GRPMultiplier,
+		"ZennyMultiplier":     cfg.GameplayOptions.ZennyMultiplier,
+		"MaterialMultiplier":  cfg.GameplayOptions.MaterialMultiplier,
+		"GCPMultiplier":       cfg.GameplayOptions.GCPMultiplier,
+		"GMaterialMultiplier": cfg.GameplayOptions.GMaterialMultiplier,
+	}
+	for name, val := range multipliers {
+		if val != 1.0 {
+			t.Errorf("%s = %v, want 1.0", name, val)
+		}
+	}
+
+	// Entrance entries should be present
+	if len(cfg.Entrance.Entries) != 6 {
+		t.Errorf("Entrance.Entries = %d, want 6", len(cfg.Entrance.Entries))
+	}
+
+	// Commands should be present
+	if len(cfg.Commands) != 12 {
+		t.Errorf("Commands = %d, want 12", len(cfg.Commands))
+	}
+
+	// Courses should be present
+	if len(cfg.Courses) != 11 {
+		t.Errorf("Courses = %d, want 11", len(cfg.Courses))
+	}
+
+	// Standard ports
+	if cfg.Sign.Port != 53312 {
+		t.Errorf("Sign.Port = %d, want 53312", cfg.Sign.Port)
+	}
+	if cfg.API.Port != 8080 {
+		t.Errorf("API.Port = %d, want 8080", cfg.API.Port)
+	}
+	if cfg.Entrance.Port != 53310 {
+		t.Errorf("Entrance.Port = %d, want 53310", cfg.Entrance.Port)
+	}
+
+	// Servers enabled by default
+	if !cfg.Sign.Enabled {
+		t.Error("Sign.Enabled should be true")
+	}
+	if !cfg.API.Enabled {
+		t.Error("API.Enabled should be true")
+	}
+	if !cfg.Channel.Enabled {
+		t.Error("Channel.Enabled should be true")
+	}
+	if !cfg.Entrance.Enabled {
+		t.Error("Entrance.Enabled should be true")
+	}
+
+	// Database defaults
+	if cfg.Database.Host != "localhost" {
+		t.Errorf("Database.Host = %q, want localhost", cfg.Database.Host)
+	}
+	if cfg.Database.Port != 5432 {
+		t.Errorf("Database.Port = %d, want 5432", cfg.Database.Port)
+	}
+
+	// ClientMode defaults to ZZ
+	if cfg.RealClientMode != ZZ {
+		t.Errorf("RealClientMode = %v, want ZZ", cfg.RealClientMode)
+	}
+
+	// BinPath default
+	if cfg.BinPath != "bin" {
+		t.Errorf("BinPath = %q, want bin", cfg.BinPath)
+	}
+
+	// Gameplay limits
+	if cfg.GameplayOptions.MaximumNP != 100000 {
+		t.Errorf("MaximumNP = %d, want 100000", cfg.GameplayOptions.MaximumNP)
+	}
+}
+
+// TestFullConfigBackwardCompat verifies that existing full configs still load correctly.
+func TestFullConfigBackwardCompat(t *testing.T) {
+	viper.Reset()
+	dir := t.TempDir()
+	origDir, _ := os.Getwd()
+	defer func() { _ = os.Chdir(origDir) }()
+	if err := os.Chdir(dir); err != nil {
+		t.Fatal(err)
+	}
+
+	// Read the reference config (the full original config.example.json).
+	// Look in the project root (one level up from config/).
+	refPath := filepath.Join(origDir, "..", "config.reference.json")
+	refData, err := os.ReadFile(refPath)
+	if err != nil {
+		t.Skipf("config.reference.json not found at %s, skipping backward compat test", refPath)
+	}
+	writeMinimalConfig(t, dir, string(refData))
+
+	cfg, err := LoadConfig()
+	if err != nil {
+		t.Fatalf("LoadConfig() with full config error: %v", err)
+	}
+
+	// Spot-check values from the reference config
+	if cfg.GameplayOptions.HRPMultiplier != 1.0 {
+		t.Errorf("HRPMultiplier = %v, want 1.0", cfg.GameplayOptions.HRPMultiplier)
+	}
+	if cfg.Sign.Port != 53312 {
+		t.Errorf("Sign.Port = %d, want 53312", cfg.Sign.Port)
+	}
+	if len(cfg.Entrance.Entries) != 6 {
+		t.Errorf("Entrance.Entries = %d, want 6", len(cfg.Entrance.Entries))
+	}
+	if len(cfg.Commands) != 12 {
+		t.Errorf("Commands = %d, want 12", len(cfg.Commands))
+	}
+	if cfg.GameplayOptions.MaximumNP != 100000 {
+		t.Errorf("MaximumNP = %d, want 100000", cfg.GameplayOptions.MaximumNP)
+	}
+}
+
+// TestSingleFieldOverride verifies that overriding one field in a dot-notation
+// section doesn't clobber other fields' defaults.
+func TestSingleFieldOverride(t *testing.T) {
+	viper.Reset()
+	dir := t.TempDir()
+	origDir, _ := os.Getwd()
+	defer func() { _ = os.Chdir(origDir) }()
+	if err := os.Chdir(dir); err != nil {
+		t.Fatal(err)
+	}
+
+	writeMinimalConfig(t, dir, `{
+		"Database": { "Password": "test" },
+		"GameplayOptions": { "HRPMultiplier": 2.0 }
+	}`)
+
+	cfg, err := LoadConfig()
+	if err != nil {
+		t.Fatalf("LoadConfig() error: %v", err)
+	}
+
+	// Overridden field
+	if cfg.GameplayOptions.HRPMultiplier != 2.0 {
+		t.Errorf("HRPMultiplier = %v, want 2.0", cfg.GameplayOptions.HRPMultiplier)
+	}
+
+	// Other multipliers should retain defaults
+	if cfg.GameplayOptions.SRPMultiplier != 1.0 {
+		t.Errorf("SRPMultiplier = %v, want 1.0 (should retain default)", cfg.GameplayOptions.SRPMultiplier)
+	}
+	if cfg.GameplayOptions.ZennyMultiplier != 1.0 {
+		t.Errorf("ZennyMultiplier = %v, want 1.0 (should retain default)", cfg.GameplayOptions.ZennyMultiplier)
+	}
+	if cfg.GameplayOptions.GCPMultiplier != 1.0 {
+		t.Errorf("GCPMultiplier = %v, want 1.0 (should retain default)", cfg.GameplayOptions.GCPMultiplier)
+	}
+}