Feature: expose a machine-readable capability descriptor for a deployment #639

An integrator pointing at a deployment has to discover its capabilities by reading docs or trying calls: which credential types are supported, which issuers are registered, what the contract versions are, whether an indexer is available, whether sponsorship is configured, and which network it is on.

/api/ready reports dependency health and #423 added contract versions, but there is no single descriptor an integrator or the SDK can fetch to configure itself.

Scope

• Add a well-known capability endpoint returning: network, contract IDs and versions, supported credential types, registered issuer summary, indexer URL if public, whether sponsored submission is available, and the SDK version range known to work.
• Have the SDK optionally bootstrap from it so an integrator supplies one URL rather than a set of contract IDs.
• Keep it public and cacheable; it contains nothing sensitive.
• Tie it to the deployments registry in No published contract address registry for deployed networks #518 so the published record and the live descriptor agree.

Acceptance

• A deployment exposes a machine-readable capability descriptor, the SDK can bootstrap from it, and it agrees with the published deployments record.