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/agents/sisyphus.ts

@@ -539,7 +539,6 @@ export function createSisyphusAgent(
     call_omo_agent: "deny",
   } as AgentConfig["permission"];
   const base = {
-    name: "Sisyphus (Ultraworker)",
     description:
       "Powerful AI orchestrator. Plans obsessively with todos, assesses search complexity before exploration, delegates strategically via category+skills combinations. Uses explore for internal code (parallel-friendly), librarian for external docs. (Sisyphus - OhMyOpenCode)",
     mode: MODE,