Improve first-run empty state and transition into the first post

[benvinegar]

Summary

Improve the empty workspace experience so first-run users see clearer onboarding and the page transitions automatically once the first agent post arrives.

Motivation

A fresh sideshow instance should explain what to do next, show the connect-an-agent instructions inline, and then reward the user immediately when an agent publishes by replacing the empty state with real content.

Sketch

• Keep the polished connect instructions visible in the empty state.
• Add a lightweight "waiting for first post" cue.
• Poll or subscribe while the workspace is empty, then stop once a post exists.
• Replace the empty state with the normal recent-posts/session view without requiring a manual refresh.
• Avoid polling when the tab is hidden or once content exists.

Acceptance criteria

• A brand-new workspace shows useful connect guidance inline.
• Publishing the first post updates the UI automatically.
• Polling/subscription work is disabled once the workspace is no longer empty.
• Read-only/public views do not show write/connect affordances.