feat(desktop): preserve Side Conversations across linked-session navigation #4494

Problem

Desktop Side Conversations are removed whenever navigation changes the active Session away from their source Session. This also happens when the destination is a linked child/Sub Agent Session belonging to the same unit of work.

For example:

1. Open a Side Conversation from a parent Session.
2. Keep a draft or start a Turn in the Side Conversation.
3. Open one of the parent Session's linked child/Sub Agent Sessions.
4. Return to the parent Session.

The Side Conversation has been closed and its temporary fork removed. A running answer, draft, and conversation history can no longer be resumed from the parent.

This is consistent with the current temporary lifecycle rather than an implementation regression: the Desktop controller removes panels whose sourceSessionId differs from the active Session, and the Side Conversation documentation specifies cleanup when the owning Session changes. However, a linked child is still part of the same work, so treating that navigation like switching to an unrelated Session makes inspection of delegated work unnecessarily destructive.

Desired outcome

Preserve a source Session's Side Conversations while the user navigates between that source and its linked descendant Sessions.

• The Side Conversation remains owned by its original source Session; it is not reparented to the child.
• Its draft, transcript, and running Turn remain intact while a linked child is open.
• Returning to the source restores the same Side Conversation tabs.
• Explicitly closing the tab keeps the current cleanup behavior.
• Navigating to an unrelated Session may keep the current temporary cleanup behavior.
• Removing the source Session still cleans up its temporary Side Conversations.
• This does not make Side Conversations durable across restart, widen their permissions, synchronize their model with later parent changes, or promote them into ordinary Sessions.

This should be tracked as a focused lifecycle slice under #4331.

Alternatives or workarounds

• Return to the parent before opening or continuing a Side Conversation. This prevents simultaneous inspection of linked child work.
• Promote the Side Conversation to a durable ordinary Session. That is a broader product decision and is unnecessary for navigation within one linked Session family.
• Retain every Side Conversation globally. That would change the temporary lifecycle for unrelated work and is broader than needed.
• Reparent the Side Conversation to the active child. That would change lineage and cleanup ownership and should be avoided.