morning_briefing: flat file storage at sol/briefing.md
Change briefing storage from sol/briefing/{day}.md to sol/briefing.md
(same pattern as pulse.md). The day is in the YAML frontmatter. Fixes
PermissionError when cortex agent writes to a new subdirectory.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>