Create hosted channels and show workspace status in the app #177

Make hosted channels usable from the app after the deployed pilot #176 establishes the current runtime behavior. The CLI already supports ace new --hosted <url>; the app currently creates local channels.

Acceptance

• Let the team configure/select its deployed channel service through an appropriate owner setup flow. Keep secrets in the supported credential store and explain missing service/model setup without exposing credentials.
• Create a hosted channel inside the selected project, using an available checkout on this host as its workspace. Preserve project-first behavior: Cmd+O adds a project; channel creation remains a separate action.
• Show where the channel is hosted and which host is its workspace. Distinguish channel availability from workspace connection state, with useful connecting/offline/reconnecting feedback and recovery.
• Keep channel selection, chat/history, sharing, model choice and local workspace tools consistent with existing channel UI and shared protocol types. An unavailable workspace must not be presented as an unavailable transcript merely because local execution is disconnected; coordinate the lifecycle work in [Meta] Run hosted channels in the cloud with local workspaces #13.
• Verify creation, first workspace connection, failure/retry and reconnect using a deployed service, real model and actual native app. Confirm project/lane files remain on the selected workspace.

The team deploys the service; Ace does not operate hosting. Keep the tailnet as the sole authority for membership/collaboration and preserve the existing collaborator-agent switch. This issue does not add machine handoff (#49) or a separate cloud filesystem.

Priority 2 under #13. Depends on the pilot's findings, not on every distributed-work item in #58.