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/hephaestus.ts

@@ -633,7 +633,6 @@ export function createHephaestusAgent(
     : buildHephaestusPrompt([], tools, skills, categories, useTaskSystem);
 
   return {
-    name: "Hephaestus (Deep Agent)",
     description:
       "Autonomous Deep Worker - goal-oriented execution with GPT 5.2 Codex. Explores thoroughly before acting, uses explore/librarian agents for comprehensive context, completes tasks end-to-end. Inspired by AmpCode deep mode. (Hephaestus - OhMyOpenCode)",
     mode: MODE,