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"
 

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");

src/agents/hephaestus/agent.ts

@@ -13,7 +13,7 @@ import { buildHephaestusPrompt as buildGptPrompt } from "./gpt";
 import { buildHephaestusPrompt as buildGpt53CodexPrompt } from "./gpt-5-3-codex";
 import { buildHephaestusPrompt as buildGpt54Prompt } from "./gpt-5-4";
 
-const MODE: AgentMode = "all";
+const MODE: AgentMode = "primary";
 
 export type HephaestusPromptSource = "gpt-5-4" | "gpt-5-3-codex" | "gpt";
 

src/agents/hephaestus/gpt-5-3-codex.ts

@@ -21,7 +21,7 @@ import {
   buildAntiDuplicationSection,
   categorizeTools,
 } from "../dynamic-agent-prompt-builder";
-const MODE: AgentMode = "all";
+const MODE: AgentMode = "primary";
 
 function buildTodoDisciplineSection(useTaskSystem: boolean): string {
   if (useTaskSystem) {

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",