feat(agents): wire Gemini prompt routing into Sisyphus-Junior, Atlas, Prometheus

Add 'gemini' to prompt source types and route Gemini models to new
Gemini-optimized prompts via isGeminiModel detection. Update barrel
exports for all 3 agent modules. All existing tests pass.

src/agents/atlas/agent.ts

@@ -6,12 +6,13 @@
  *
  * Routing:
  * 1. GPT models (openai/*, github-copilot/gpt-*) → gpt.ts (GPT-5.2 optimized)
- * 2. Default (Claude, etc.) → default.ts (Claude-optimized)
+ * 2. Gemini models (google/*, google-vertex/*) → gemini.ts (Gemini-optimized)
+ * 3. Default (Claude, etc.) → default.ts (Claude-optimized)
  */
 
 import type { AgentConfig } from "@opencode-ai/sdk"
 import type { AgentMode, AgentPromptMetadata } from "../types"
-import { isGptModel } from "../types"
+import { isGptModel, isGeminiModel } from "../types"
 import type { AvailableAgent, AvailableSkill, AvailableCategory } from "../dynamic-agent-prompt-builder"
 import { buildCategorySkillsDelegationGuide } from "../dynamic-agent-prompt-builder"
 import type { CategoryConfig } from "../../config/schema"
@@ -20,6 +21,7 @@ import { createAgentToolRestrictions } from "../../shared/permission-compat"
 
 import { getDefaultAtlasPrompt } from "./default"
 import { getGptAtlasPrompt } from "./gpt"
+import { getGeminiAtlasPrompt } from "./gemini"
 import {
   getCategoryDescription,
   buildAgentSelectionSection,
@@ -30,7 +32,7 @@ import {
 
 const MODE: AgentMode = "primary"
 
-export type AtlasPromptSource = "default" | "gpt"
+export type AtlasPromptSource = "default" | "gpt" | "gemini"
 
 /**
  * Determines which Atlas prompt to use based on model.
@@ -39,6 +41,9 @@ export function getAtlasPromptSource(model?: string): AtlasPromptSource {
   if (model && isGptModel(model)) {
     return "gpt"
   }
+  if (model && isGeminiModel(model)) {
+    return "gemini"
+  }
   return "default"
 }
 
@@ -58,6 +63,8 @@ export function getAtlasPrompt(model?: string): string {
   switch (source) {
     case "gpt":
       return getGptAtlasPrompt()
+    case "gemini":
+      return getGeminiAtlasPrompt()
     case "default":
     default:
       return getDefaultAtlasPrompt()

src/agents/atlas/index.ts

@@ -1,5 +1,6 @@
 export { ATLAS_SYSTEM_PROMPT, getDefaultAtlasPrompt } from "./default"
 export { ATLAS_GPT_SYSTEM_PROMPT, getGptAtlasPrompt } from "./gpt"
+export { ATLAS_GEMINI_SYSTEM_PROMPT, getGeminiAtlasPrompt } from "./gemini"
 export {
   getCategoryDescription,
   buildAgentSelectionSection,