Adopt refuses an idle-looking session whose registry status is "shell" #609

Problem

A user clicks Adopt on an outside Claude session that the sidebar lists as Idle. Relay answers "Can't adopt: the session is not idle (registry_busy)". The session has no turn running. It has a background shell running. Interrupt and Adopt works.

Root cause

Claude's session registry reports status: "shell" while a background command runs between turns. Check in internal/sessions/adopt/adopt_claude.go accepts only idle (case reg.Status != "idle" gives VetoRegistryBusy). The sidebar row derives "Idle" from a different reading, so the row and the check disagree.

Evidence: a 2s log of the registry entry showed busy (turn), then shell for about 43s with no turn running, then busy. An Adopt click during the shell stretch failed with registry_busy.

Fix

• Treat shell like idle in the registry test. Adoption stops the process and its shells anyway. The other vetoes (stale status, open tool call, working child) stay.
• Or hide Adopt for a shell session. The row and the check must read one status.
• Name the veto in the toast: "a background command is running", not "not idle".

Acceptance criteria

• Adopt succeeds on a session whose registry status is shell and has no open turn.
• The sidebar row and the adopt check give the same verdict for every registry status.
• The toast names the actual veto.

What I'd notice

Adopt works on a session that is only waiting on background commands.