feat: add writing-agent subagent for long-form content creation

- Add writing-agent to agents.json with subagent mode
- Configure temperature 0.8 for creative but structured writing
- Set triggers: write, draft, compose, document, report
- Grant write access to ~/CODEX for document creation
- Add to Chiron's task.permission allowlist for delegation
- Create comprehensive system prompt with document patterns and workflows

agents/agents.json

@@ -55,6 +55,7 @@
         "athena": "allow",
         "strategy-agent": "allow",
         "brainstorming-agent": "allow",
+        "writing-agent": "allow",
         "chiron-forge": "allow",
         "meeting-facilitator-agent": "allow",
         "productivity-coach-agent": "allow"