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/sisyphus.ts

@@ -12,7 +12,7 @@ import {
 import { buildGpt54SisyphusPrompt } from "./sisyphus/gpt-5-4";
 import { buildTaskManagementSection } from "./sisyphus/default";
 
-const MODE: AgentMode = "all";
+const MODE: AgentMode = "primary";
 export const SISYPHUS_PROMPT_METADATA: AgentPromptMetadata = {
   category: "utility",
   cost: "EXPENSIVE",