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/momus.ts

@@ -197,6 +197,7 @@ export function createMomusAgent(model: string): AgentConfig {
   ])
 
   const base = {
+    name: "Momus (Plan Critic)",
     description:
       "Expert reviewer for evaluating work plans against rigorous clarity, verifiability, and completeness standards. (Momus - OhMyOpenCode)",
     mode: MODE,