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/database/querier.go

@@ -349,6 +349,11 @@ type sqlcQuerier interface {
 	// snapshot collection. Uses updated_at so that long-running chats
 	// still appear in each snapshot window while they are active.
 	GetChatsUpdatedAfter(ctx context.Context, updatedAfter time.Time) ([]GetChatsUpdatedAfterRow, error)
+	// Fetches child chats of the given parents, optionally filtered by
+	// archive state (NULL = all, true/false = match). The archive
+	// invariant (parent archived implies child archived) is enforced
+	// at write time, not here.
+	GetChildChatsByParentIDs(ctx context.Context, arg GetChildChatsByParentIDsParams) ([]GetChildChatsByParentIDsRow, error)
 	GetConnectionLogsOffset(ctx context.Context, arg GetConnectionLogsOffsetParams) ([]GetConnectionLogsOffsetRow, error)
 	GetCryptoKeyByFeatureAndSequence(ctx context.Context, arg GetCryptoKeyByFeatureAndSequenceParams) (CryptoKey, error)
 	GetCryptoKeys(ctx context.Context) ([]CryptoKey, error)