Add bundled pi default and session token reporting

src/agents/defaults.ts

@@ -0,0 +1,5 @@
+// Defaults for agent metadata when upstream does not supply them.
+// Model id uses pi-ai's built-in Anthropic catalog.
+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;