refactor: remove file-based configuration, use env vars only

Remove the entire pkg/config package (Viper-based YAML config file
loader) and simplify internal/config to read all settings directly from
environment variables via os.Getenv(). This eliminates the spurious
"Failed to load config" log messages that appeared when no config.yaml
file was present.

- Delete pkg/config/ (YAML loader, resolver, manager, tests)
- Delete configs/config.yaml.example
- Simplify internal/config helper functions to use os.Getenv() with
  defaults instead of falling back to pkgconfig
- Update tests to set env vars directly instead of creating in-memory
  YAML config files via afero
- Remove afero, cloud.google.com/*, aws-sdk-go dependencies from go.mod
- Update README: document env-var-only configuration, remove YAML/Viper
  references
- Keep godotenv/autoload for .env file convenience in local development

closes #27

Dockerfile

@@ -34,7 +34,6 @@ RUN set -eux; \
 
 # Copy go module files and download dependencies
 COPY go.mod go.sum ./
-COPY pkg/config/go.mod pkg/config/go.sum ./pkg/config/
 RUN go mod download
 
 # Copy source code