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/shared/agent-display-names.ts

@@ -35,4 +35,20 @@ export function getAgentDisplayName(configKey: string): string {
   
   // Unknown agent: return original key
   return configKey
+}
+
+const REVERSE_DISPLAY_NAMES: Record<string, string> = Object.fromEntries(
+  Object.entries(AGENT_DISPLAY_NAMES).map(([key, displayName]) => [displayName.toLowerCase(), key]),
+)
+
+/**
+ * Resolve an agent name (display name or config key) to its lowercase config key.
+ * "Atlas (Plan Executor)" → "atlas", "atlas" → "atlas", "unknown" → "unknown"
+ */
+export function getAgentConfigKey(agentName: string): string {
+  const lower = agentName.toLowerCase()
+  const reversed = REVERSE_DISPLAY_NAMES[lower]
+  if (reversed !== undefined) return reversed
+  if (AGENT_DISPLAY_NAMES[lower] !== undefined) return lower
+  return lower
 }