Remap config.agent keys to display names at output boundary

Use display names as config.agent keys so opencode shows proper names in UI
(Tab/@ menu). Key remapping happens after all agents are assembled but before
reordering, via remapAgentKeysToDisplayNames().

- agent-config-handler: set default_agent to display name, add key remapping
- agent-key-remapper: new module to transform lowercase keys to display names
- agent-priority-order: CORE_AGENT_ORDER uses display names
- tool-config-handler: look up agents by config key via agentByKey() helper

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

@@ -1,4 +1,11 @@
-const CORE_AGENT_ORDER = ["sisyphus", "hephaestus", "prometheus", "atlas"] as const;
+import { getAgentDisplayName } from "../shared/agent-display-names";
+
+const CORE_AGENT_ORDER = [
+  getAgentDisplayName("sisyphus"),
+  getAgentDisplayName("hephaestus"),
+  getAgentDisplayName("prometheus"),
+  getAgentDisplayName("atlas"),
+] as const;
 
 export function reorderAgentsByPriority(
   agents: Record<string, unknown>,