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.

docs/configurations.md

@@ -63,7 +63,7 @@ Override built-in agent settings:
       "model": "anthropic/claude-haiku-4-5",
       "temperature": 0.5
     },
-    "frontend-ui-ux-engineer": {
+    "multimodal-looker": {
       "disable": true
     }
   }
@@ -116,11 +116,11 @@ Or disable via `disabled_agents` in `~/.config/opencode/oh-my-opencode.json` or
 
 ```json
 {
-  "disabled_agents": ["oracle", "frontend-ui-ux-engineer"]
+  "disabled_agents": ["oracle", "multimodal-looker"]
 }
 ```
 
-Available agents: `oracle`, `librarian`, `explore`, `frontend-ui-ux-engineer`, `document-writer`, `multimodal-looker`
+Available agents: `oracle`, `librarian`, `explore`, `multimodal-looker`
 
 ## Built-in Skills
 

docs/guide/installation.md

@@ -136,8 +136,6 @@ The `opencode-antigravity-auth` plugin uses different model names than the built
 ```json
 {
   "agents": {
-    "frontend-ui-ux-engineer": { "model": "google/antigravity-gemini-3-pro-high" },
-    "document-writer": { "model": "google/antigravity-gemini-3-flash" },
     "multimodal-looker": { "model": "google/antigravity-gemini-3-flash" }
   }
 }