Mobile client creates a duplicate session on every reconnect to a remote session; auto-archive on disconnect no longer fires

**What happens**

Connecting from the mobile client to an existing remote session creates a *new*
session record with the same title instead of attaching to the existing one.
Previously, disconnecting auto-archived the duplicate, so this was invisible.
That auto-archive no longer happens, so every login/reconnect cycle leaves a
permanent duplicate behind and the session list on the phone grows without bound.

**Expected**

Reconnecting attaches to the existing session, or — failing that — the duplicate
shell is archived on disconnect as it used to be.

**Evidence**

From a Claude Code desktop session on the host machine, `ListAgents` reports 26
peer sessions labeled `Remote Control · offline`. Grouped by title:

- one title appears 9 times (9 distinct session refs)
- two more titles appear 3 times each
- the remaining 11 titles appear once

So 26 entries correspond to roughly 14 real conversations plus ~12 duplicate
shells. The duplicates carry distinct session refs but identical titles.

These sessions have no record on the host machine: the desktop session store
returns only 5 sessions for this project even with archived included, none of
them matching the duplicated titles, and there are no local transcripts for them
under `~/.claude/projects/`. The duplicates therefore exist only account-side and
cannot be cleaned up from the desktop client — only from the phone, by hand.

**Environment**

- Host: macOS (Darwin 25.6.0), Claude Code desktop app
- Client creating the duplicates: Claude mobile app, Remote Control
- Behavior change: auto-archive-on-disconnect worked previously, stopped at some
point (exact version unknown)

No source file or test is named. Start by tracing the mobile Remote Control reconnect flow and disconnect path, using ListAgents and the account-side session records as the observable entry points; done means reconnecting attaches to the existing session or the duplicate shell is archived, without permanent duplicates.

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