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/features/builtin-commands/templates/init-deep.ts

@@ -236,11 +236,11 @@ AGENTS_LOCATIONS = [
 
 ### Subdirectory AGENTS.md (Parallel)
 
-Launch document-writer agents for each location:
+Launch writing tasks for each location:
 
 \`\`\`
 for loc in AGENTS_LOCATIONS (except root):
-  delegate_task(agent="document-writer", prompt=\\\`
+  delegate_task(category="writing", prompt=\\\`
     Generate AGENTS.md for: \${loc.path}
     - Reason: \${loc.reason}
     - 30-80 lines max