feat(security): commandsSyncKey is a required channel-config field (Guild C-2)

The slash-command sync secret now comes from
channels.fabric.commandsSyncKey (configSchema marks it required) and
is no longer read from FABRIC_COMMANDS_SYNC_KEY env. command-sync
resolves it from config and threads it into client.syncCommands;
when absent, sync is skipped with a clear warning. README updated.

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

openclaw.plugin.json

@@ -39,6 +39,11 @@
             "type": "string",
             "description": "Fabric Center API base, e.g. http://localhost:7001/api"
           },
+          "commandsSyncKey": {
+            "type": "string",
+            "minLength": 1,
+            "description": "Shared secret that must equal the guild's FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY. Required to register the slash-command catalog (Guild C-2). Read it from the guild via: docker exec fabric-backend-guild node dist/cli/print-commands-sync-key.js"
+          },
           "dmSecurity": { "type": "string" },
           "dmPolicy": { "type": "string" },
           "enabled": { "type": "boolean" },
@@ -59,10 +64,12 @@
               }
             }
           }
-        }
+        },
+        "required": ["commandsSyncKey"]
       },
       "uiHints": {
-        "centerApiBase": { "label": "Center API base" }
+        "centerApiBase": { "label": "Center API base" },
+        "commandsSyncKey": { "label": "Commands sync key" }
       }
     }
   }