Fixes#16092
The app-server-backed TUI could accumulate ghost subagent entries in
`/agent` after resume/backfill flows. Some of those rows were no longer
live according to the backend, but still appeared selectable in the
picker and could open as blank threads.
*Cause*
Unlike the legacy tui behavior, tui_app_server was creating local
picker/replay state for subagents discovered through metadata refresh
and loaded-thread backfill, even when no real local session or
transcript had been attached. That let stale ids survive in the picker
as if they were replayable threads.
*Fix*
Stop creating empty local thread channels during subagent metadata
hydration and loaded-thread backfill.
When opening /agent, prune metadata-only entries that thread/read
reports as terminally unavailable.
When selecting a discovered subagent that is still live but not yet
locally attached, materialize a real local session on demand from
thread/read instead of falling back to an empty replay state.
38e648ca67