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/src/cli.ts

@@ -2,6 +2,7 @@
 
 import { createRequire } from 'module';
 import { Command } from 'commander';
+import { registerBrainCommand } from '@mosaicstack/brain';
 import { registerQualityRails } from '@mosaicstack/quality-rails';
 import { registerAgentCommand } from './commands/agent.js';
 import { registerMissionCommand } from './commands/mission.js';
@@ -314,6 +315,10 @@ registerAgentCommand(program);
 
 registerMissionCommand(program);
 
+// ─── brain ──────────────────────────────────────────────────────────────
+
+registerBrainCommand(program);
+
 // ─── quality-rails ──────────────────────────────────────────────────────
 
 registerQualityRails(program);