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
