feat(brain): add registerBrainCommand for mosaic brain CLI surface

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>

packages/mosaic/package.json

@@ -27,6 +27,7 @@
     "test": "vitest run --passWithNoTests"
   },
   "dependencies": {
+    "@mosaicstack/brain": "workspace:*",
     "@mosaicstack/config": "workspace:*",
     "@mosaicstack/forge": "workspace:*",
     "@mosaicstack/macp": "workspace:*",