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

.github/workflows/release.yml

@@ -43,6 +43,7 @@ jobs:
         mkdir -p staging
         cp ${{ matrix.binary }} staging/
         cp config.example.json staging/
+        cp config.reference.json staging/
         cp -r www/ staging/www/
         cp -r savedata/ staging/savedata/
         # Schema is now embedded in the binary via server/migrations/