fix(agents): make sisyphus, hephaestus, atlas primary-only (not callable as subagents)

These agents should only be usable as primary session agents, not as
subagent targets via call_omo_agent/task(). Previously MODE was 'all'
which allowed them to be spawned as subagents, leading to confusing
behavior (e.g. Atlas delegating to Hephaestus as a subagent).

Subagent-callable agents remain: oracle, explore, librarian,
multimodal-looker, metis, momus, sisyphus-junior.

Note: Prometheus is not a BuiltinAgentName and is only invoked via
slash commands, so no change needed there.

src/agents/hephaestus/agent.test.ts

@@ -170,7 +170,7 @@ describe("createHephaestusAgent", () => {
 
     // then
     expect(config).toHaveProperty("description");
-    expect(config).toHaveProperty("mode", "all");
+    expect(config).toHaveProperty("mode", "primary");
     expect(config).toHaveProperty("model", "openai/gpt-5.4");
     expect(config).toHaveProperty("maxTokens", 32000);
     expect(config).toHaveProperty("prompt");