Make native Muse subscription available without experimental opt-in #1991

Requested behavior

Make the existing native muse-subscription device-login route usable without JCODE_EXPERIMENTAL_MUSE_SUBSCRIPTION=1 and remove experimental labels from login, auth status and provider selection.

Evidence

• The native route merged in feat(provider): native Muse subscription login without the Muse CLI #1886 and uses Jcode-owned credentials, model discovery and the Responses transport, not a Muse CLI bridge.
• The operator reports using this route successfully in their separate jcodemuse installation and requested removal of the local feature gate.
• The current opt-in check blocks otherwise valid native credentials and login entrypoints.

Safety boundary

Removing a Jcode feature flag does not establish Meta endorsement or guaranteed subscription settlement. Preserve the active-subscription and payment-required checks, credential storage/refresh protections, redirect/host validation, and separation from pay-as-you-go meta-muse credentials. Retain a clear third-party-client/billing notice.

Acceptance

Run protocol and catalog tests without the opt-in flag, test legacy flag values, test the native provider with saved credentials and no flag, and keep contributor-model selection deliberate. No changes to the unrelated harness PR #1892.