feat(agent): enforce provider/model and identity defaults

src/agents/defaults.ts

@@ -1,5 +1,6 @@
 // Defaults for agent metadata when upstream does not supply them.
 // Model id uses pi-ai's built-in Anthropic catalog.
+export const DEFAULT_PROVIDER = "anthropic";
 export const DEFAULT_MODEL = "claude-opus-4-5";
 // Context window: Opus 4.5 supports ~200k tokens (per pi-ai models.generated.ts).
 export const DEFAULT_CONTEXT_TOKENS = 200_000;