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-key-remapper.ts

@@ -0,0 +1,18 @@
+import { AGENT_DISPLAY_NAMES } from "../shared/agent-display-names"
+
+export function remapAgentKeysToDisplayNames(
+  agents: Record<string, unknown>,
+): Record<string, unknown> {
+  const result: Record<string, unknown> = {}
+
+  for (const [key, value] of Object.entries(agents)) {
+    const displayName = AGENT_DISPLAY_NAMES[key]
+    if (displayName && displayName !== key) {
+      result[displayName] = value
+    } else {
+      result[key] = value
+    }
+  }
+
+  return result
+}