Summary

Add GOG_KEYRING_SERVICE_NAME as an environment variable override for the keyring ServiceName.

This keeps the default behavior unchanged (gogcli), but allows Linux/KWallet users to point gog at a different wallet/collection name when needed, for example:

export GOG_KEYRING_SERVICE_NAME=kdewallet

Changes

• add GOG_KEYRING_SERVICE_NAME support in internal/secrets/store.go
• keep the default service name as gogcli when the env var is unset
• add tests covering the default and override behavior
• document the new env var in README.md and docs/spec.md

Test plan

• GOCACHE=/tmp/go-build go test ./internal/secrets

User-facing changes

• new env var: GOG_KEYRING_SERVICE_NAME
• no behavior change unless the env var is set