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/atlas/agent.ts

@@ -99,6 +99,7 @@ export function createAtlasAgent(ctx: OrchestratorContext): AgentConfig {
   ])
 
   const baseConfig = {
+    name: "Atlas (Plan Executor)",
     description:
       "Orchestrates work via task() to complete ALL tasks in a todo list until fully done. (Atlas - OhMyOpenCode)",
     mode: MODE,