OS keychain backend for secretx.

Integration test status

Unit tests (URI parsing, error mapping) pass without a keyring daemon. The integration test (SECRETX_KEYRING_INTEGRATION_TESTS=1) requires a running desktop keyring daemon (macOS Keychain, Windows Credential Manager, or gnome-keyring-daemon / KWallet on Linux). On a headless Linux server put succeeds but get returns NotFound — do not run in CI without a keyring daemon. Integration-tested on: macOS, Windows (not yet). Linux desktop: not yet.

URI: secretx://keyring/<service>/<account>

# async fn example() -> Result<(), secretx_core::SecretError> {
use secretx_keyring::KeyringBackend;
use secretx_core::SecretStore;

let store = KeyringBackend::from_uri("secretx://keyring/my-app/api-key")?;
let value = store.get().await?;
# Ok(())
# }