fix: getenv treats an explicitly-empty value as unset (#85)

Fixes #69

## Why
`getenv` returned the fallback whenever `os.Getenv` was empty, so an intentionally-empty env var could not override a non-empty default.

## Changes
- Use `os.LookupEnv` to distinguish unset from set-but-empty.

## Validation
- `make e2e` passes.

Reviewed-on: #85
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/config/env.go

@@ -65,7 +65,7 @@ func Load() (*Config, error) {
 }
 
 func getenv(key, fallback string) string {
-	if v := os.Getenv(key); v != "" {
+	if v, ok := os.LookupEnv(key); ok {
 		return v
 	}
 	return fallback