refactor(agents): remove document-writer agent in favor of writing category

- Delete document-writer.ts agent file
- Remove from types, schema, utils, index exports
- Remove tool restrictions entry
- Remove migration mappings
- Update atlas.ts to use category="writing" instead of agent="document-writer"
- Update init-deep command template
- Update all documentation (AGENTS.md, README.*, docs/)
- Regenerate schema.json

document-writer functionality is now handled via delegate_task with
category="writing" which uses the writing category's model config.

src/shared/migration.ts

@@ -17,8 +17,6 @@ export const AGENT_NAME_MAP: Record<string, string> = {
   oracle: "oracle",
   librarian: "librarian",
   explore: "explore",
-  "frontend-ui-ux-engineer": "frontend-ui-ux-engineer",
-  "document-writer": "document-writer",
   "multimodal-looker": "multimodal-looker",
   "orchestrator-sisyphus": "atlas",
 }
@@ -28,8 +26,6 @@ export const BUILTIN_AGENT_NAMES = new Set([
   "oracle",
   "librarian",
   "explore",
-  "frontend-ui-ux-engineer",
-  "document-writer",
   "multimodal-looker",
   "Metis (Plan Consultant)",
   "Momus (Plan Reviewer)",