refactor(agents/atlas): remove unused barrel exports

Clean up unused re-exports from atlas agents barrel file

Remove 12 unused exports identified by knip analysis

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/atlas/index.ts

@@ -1,15 +1,2 @@
-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,
-  buildCategorySection,
-  buildSkillsSection,
-  buildDecisionMatrix,
-} from "./prompt-section-builder"
-
-export { createAtlasAgent, getAtlasPromptSource, getAtlasPrompt, atlasPromptMetadata } from "./agent"
+export { createAtlasAgent, atlasPromptMetadata } from "./agent"
 export type { AtlasPromptSource, OrchestratorContext } from "./agent"
-
-export { isGptModel } from "../types"