Revert name fields from agent configs, add getAgentConfigKey reverse lookup

Remove crash-causing name fields from 6 agent configs (sisyphus, hephaestus,
atlas, metis, momus, prometheus). The name field approach breaks opencode
because Agent.get(agent.name) uses name as lookup key.

Add getAgentConfigKey() to agent-display-names.ts for resolving display names
back to lowercase config keys (e.g. 'Atlas (Plan Executor)' -> 'atlas').

src/plugin-handlers/prometheus-agent-config-builder.ts

@@ -66,7 +66,6 @@ export async function buildPrometheusAgentConfig(params: {
     params.pluginPrometheusOverride?.maxTokens ?? categoryConfig?.maxTokens;
 
   const base: Record<string, unknown> = {
-    name: "Prometheus (Plan Builder)",
     ...(resolvedModel ? { model: resolvedModel } : {}),
     ...(variantToUse ? { variant: variantToUse } : {}),
     mode: "all",