Add display names to all core agents via name field

Sisyphus (Ultraworker), Hephaestus (Deep Agent), Prometheus (Plan Builder),
Atlas (Plan Executor), Metis (Plan Consultant), Momus (Plan Critic).

Requires opencode fix: Agent.get() fallback to name-based lookup when key
lookup fails, since opencode stores agent.name in messages and reuses it
for subsequent Agent.get() calls.

src/agents/hephaestus.ts

@@ -633,6 +633,7 @@ 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,