feat(agent-priority): inject order field for deterministic agent Tab cycling

Inject an explicit `order` field (1-4) into the four core agents
(Sisyphus, Hephaestus, Prometheus, Atlas) via reorderAgentsByPriority().
This pre-empts OpenCode's alphabetical agent sorting so the intended
Tab cycle order is preserved once OpenCode merges order field support
(anomalyco/opencode#19127).

Refs anomalyco/opencode#7372

src/plugin-handlers/agent-config-handler.test.ts

@@ -60,6 +60,7 @@ describe("applyAgentConfig builtin override protection", () => {
     name: "Builtin Sisyphus",
     prompt: "builtin prompt",
     mode: "primary",
+    order: 1,
   }
 
   const builtinOracleConfig: AgentConfig = {

src/plugin-handlers/agent-priority-order.ts

@@ -1,11 +1,21 @@
 import { getAgentDisplayName } from "../shared/agent-display-names";
 
-const CORE_AGENT_ORDER = [
-  getAgentDisplayName("sisyphus"),
-  getAgentDisplayName("hephaestus"),
-  getAgentDisplayName("prometheus"),
-  getAgentDisplayName("atlas"),
-] as const;
+const CORE_AGENT_ORDER: ReadonlyArray<{ displayName: string; order: number }> = [
+  { displayName: getAgentDisplayName("sisyphus"), order: 1 },
+  { displayName: getAgentDisplayName("hephaestus"), order: 2 },
+  { displayName: getAgentDisplayName("prometheus"), order: 3 },
+  { displayName: getAgentDisplayName("atlas"), order: 4 },
+];
+
+function injectOrderField(
+  agentConfig: unknown,
+  order: number,
+): unknown {
+  if (typeof agentConfig === "object" && agentConfig !== null) {
+    return { ...agentConfig, order };
+  }
+  return agentConfig;
+}
 
 export function reorderAgentsByPriority(
   agents: Record<string, unknown>,
@@ -13,10 +23,10 @@ export function reorderAgentsByPriority(
   const ordered: Record<string, unknown> = {};
   const seen = new Set<string>();
 
-  for (const key of CORE_AGENT_ORDER) {
-    if (Object.prototype.hasOwnProperty.call(agents, key)) {
-      ordered[key] = agents[key];
-      seen.add(key);
+  for (const { displayName, order } of CORE_AGENT_ORDER) {
+    if (Object.prototype.hasOwnProperty.call(agents, displayName)) {
+      ordered[displayName] = injectOrderField(agents[displayName], order);
+      seen.add(displayName);
     }
   }