Define host-time provider input bundle for agent kits #517

Goal

Define the generic, host-time contract by which provider kits expose provider facts and neutral model catalogs to agent kits. USAi must be one implementation of the contract, not a special case in consumers.

Scope

• Define separate provider-facts and per-provider model-catalog artifacts.
• Permit optional neutral model recommendations such as primary, small, and compaction.
• Keep final harness role selection and all harness-native vocabulary in each agent kit.
• Define stable packaged and projected paths, schema versions, identifiers, validation limits, and failure behavior.
• Add a fictitious non-USAi provider fixture and CI schema validation.
• Reconcile the host-time milestone with the ownership boundary in integrations/isolation/docs/decisions/0004-neutral-model-provider-discovery.md; the runtime orchestrator remains deferred.

Acceptance criteria

• A fictitious provider validates without USAi-specific fields.
• USAi validates as an ordinary provider instance.
• Model catalogs contain no secrets, routing authority, or OpenCode-native keys.
• Provider facts contain no OpenCode-native keys.
• Recommendations are optional, provider-local model references and are explicitly advisory.
• Invalid IDs, unsafe feed-controlled strings, duplicate IDs, malformed URLs, excessive model counts, and unknown required fields fail closed.
• Validation runs in the repository CI path.
• Offline USAi catalog generation remains deterministic.

Relationships

Parent milestone: #357.

Coordinates the consumed contract currently spread across #424, #435, and #456. It does not include the runtime aggregate tracked by #425.

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