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

README.md

@@ -156,7 +156,7 @@ Edit `config.json` before starting the server. The essential settings are:
 | `BinPath` | Path to quest/scenario files |
 | `Language` | `"en"` or `"jp"` |
 
-For the full configuration reference (gameplay multipliers, debug options, Discord integration, in-game commands, entrance/channel definitions), see [config.example.json](./config.example.json) and the [Erupe Wiki](https://github.com/Mezeporta/Erupe/wiki).
+`config.example.json` is intentionally minimal — all other settings have sane defaults built into the server. For the full configuration reference (gameplay multipliers, debug options, Discord integration, in-game commands, entrance/channel definitions), see [config.reference.json](./config.reference.json) and the [Erupe Wiki](https://github.com/Mezeporta/Erupe/wiki).
 
 ## Features