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/brain/package.json

@@ -22,7 +22,8 @@
   },
   "dependencies": {
     "@mosaicstack/db": "workspace:^",
-    "@mosaicstack/types": "workspace:*"
+    "@mosaicstack/types": "workspace:*",
+    "commander": "^13.0.0"
   },
   "devDependencies": {
     "typescript": "^5.8.0",