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 = {