morning_briefing: enable write for provider sandbox compat

The OpenAI provider's read-only sandbox blocks filesystem writes from
agent tool calls. Setting write: true gives the agent permission to
persist its briefing output via sol call sol briefing --write. This
matches the pattern used by other cogitate agents that persist output
(e.g., coder.md).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>