feat(ops): CLI to print the commands-sync key (Guild C-2)

node dist/cli/print-commands-sync-key.js (npm run print:commands-key)
outputs FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY as the process sees it,
so an operator can docker-exec it on the deployed guild and copy the
value into the plugin's FABRIC_COMMANDS_SYNC_KEY. --export prints a
ready-to-paste assignment; exits 1 when unset (fallback mode).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "build": "tsc -p tsconfig.build.json",
     "start": "node dist/main.js",
+    "print:commands-key": "node dist/cli/print-commands-sync-key.js",
     "start:dev": "ts-node src/main.ts",
     "lint": "eslint 'src/**/*.ts'",
     "lint:fix": "eslint 'src/**/*.ts' --fix",