Implements task CU-05-05. Adds a registerBrainCommand(parent) export to
@mosaicstack/brain that wires brain projects/missions/tasks/conversations
subcommands onto the mosaic CLI. All DB-backed commands accept --db or
MOSAIC_DB_URL. Smoke tests assert command tree without opening a real DB.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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