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

docker/README.md

@@ -8,7 +8,7 @@
    cp config.example.json docker/config.json
    ```
 
-   Edit `docker/config.json` — set `Database.Host` to `"db"` and match the password to `docker-compose.yml` (default: `password`).
+   Edit `docker/config.json` — set `Database.Host` to `"db"` and `Database.Password` to match `docker-compose.yml` (default: `password`). The example config is minimal; see `config.reference.json` for all available options.
 
 2. Place your [quest/scenario files](https://files.catbox.moe/xf0l7w.7z) in `docker/bin/`.