feat (fabric-x): Add config-driven Fabric-X namespaces

Namespaces should be defined from the Fablo config rather than being hardcoded in the generated scripts. Research a clean way to represent Fabric-X namespaces in the shared schema without creating a separate Fabric-X schema.

Starting point: add a top-level `namespaces` section to `fablo-config.json`

The result should let users declare at least one namespace in `fablo-config.json`, generate the needed Fabric-X configuration from it, and create that namespace through the Fabric-X namespace command flow

Start with the shared schema and the top-level `namespaces` section in `fablo-config.json`, then trace how generated scripts receive Fabric-X configuration. Follow the Fabric-X namespace command flow and verify that a declared namespace is generated and created successfully.

Written by the indexing model from the issue text.