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/atlas/agent.ts

@@ -29,7 +29,7 @@ import {
   buildDecisionMatrix,
 } from "./prompt-section-builder"
 
-const MODE: AgentMode = "all"
+const MODE: AgentMode = "primary"
 
 export type AtlasPromptSource = "default" | "gpt" | "gemini"