fix: exclude subagent chats from sidebar pagination (#24404)

GetChats now returns only root chats (parent_chat_id IS NULL).
A new GetChildChatsByParentIDs query fetches children for visible
roots and embeds them in each parent's Children field. The
singular getChat endpoint does the same.

Archive invariant is one-way: parent archived implies child
archived. Parent archive/unarchive cascades via root_chat_id.
Individual child archive is permitted; child unarchive while the
parent is archived is rejected atomically (row lock on child,
re-read parent inside the transaction). Embedded children are
filtered by the caller's archive state so individually-archived
children stay hidden from active-parent views.

Gitsync MarkStale uses GetChatsByWorkspaceIDs directly;
MarkStaleParams.OwnerID removed (dead after the switch).

Frontend: buildChatTree reads from the embedded children field,
WebSocket handlers route child events into the parent's children
array, and archiving a child strips it from the parent cache.

coderd/workspaceagents.go

@@ -2046,7 +2046,6 @@ func (api *API) workspaceAgentsExternalAuth(rw http.ResponseWriter, r *http.Requ
 		//nolint:gocritic // Chat processor context required for cross-user chat lookup
 		api.gitSyncWorker.MarkStale(dbauthz.AsChatd(ctx), gitsync.MarkStaleParams{
 			WorkspaceID: workspace.ID,
-			OwnerID:     workspace.OwnerID,
 			Branch:      gitRef.Branch,
 			Origin:      gitRef.RemoteOrigin,
 			ChatID:      gitRef.ChatID,
@@ -2201,7 +2200,6 @@ func (api *API) workspaceAgentsExternalAuthListen(ctx context.Context, rw http.R
 		//nolint:gocritic // Chat processor context required for cross-user chat lookup
 		api.gitSyncWorker.MarkStale(dbauthz.AsChatd(ctx), gitsync.MarkStaleParams{
 			WorkspaceID: workspace.ID,
-			OwnerID:     workspace.OwnerID,
 			Branch:      gitRef.Branch,
 			Origin:      gitRef.RemoteOrigin,
 			ChatID:      gitRef.ChatID,