Use tabletopmancer schema registry for dice.json

[ubermanu]

Currently schema/dice.json is stored locally as a workaround to avoid an external network dependency during CI validation.

The dice schema was originally referenced from the tabletopmancer shared schema registry:

https://ubermanu.github.io/tabletopmancer/schema/dice.json

What should change

• Remove schema/dice.json from this repository
• Update schema/weapon.json and schema/class.json to reference the registry URL again
• Update the CI workflow to fetch the schema from the registry (or bundle it at build time)

Why deferred

The registry URL is not yet stable / publicly hosted in a way that works reliably in offline CI. Once the tabletopmancer schema registry is properly published and versioned, this repo should consume it as a dependency rather than maintaining a local copy.