feat(agents): add ZWSP stable-sort prefixes for core agent list ordering

Populate AGENT_LIST_SORT_PREFIXES for sisyphus/hephaestus/prometheus/atlas
so the TUI agent list renders in canonical order. Update dependent tests
to use getAgentListDisplayName() instead of hardcoded display strings.

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src/shared/agent-display-names.ts

@@ -20,7 +20,12 @@ export const AGENT_DISPLAY_NAMES: Record<string, string> = {
   "council-member": "council-member",
 }
 
-const AGENT_LIST_SORT_PREFIXES: Record<string, string> = {}
+const AGENT_LIST_SORT_PREFIXES: Record<string, string> = {
+  sisyphus: "\u200B",
+  hephaestus: "\u200B\u200B",
+  prometheus: "\u200B\u200B\u200B",
+  atlas: "\u200B\u200B\u200B\u200B",
+}
 
 function stripAgentListSortPrefix(agentName: string): string {
   return agentName.replace(/^\u200B+/, "")