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>

pnpm-lock.yaml

@@ -294,6 +294,9 @@ importers:
       '@mosaicstack/types':
         specifier: workspace:*
         version: link:../types
+      commander:
+        specifier: ^13.0.0
+        version: 13.1.0
     devDependencies:
       typescript:
         specifier: ^5.8.0
@@ -454,6 +457,9 @@ importers:
       '@clack/prompts':
         specifier: ^0.9.1
         version: 0.9.1
+      '@mosaicstack/brain':
+        specifier: workspace:*
+        version: link:../brain
       '@mosaicstack/config':
         specifier: workspace:*
         version: link:../config