feat(agent): per-upstream pi Models with isolated credential stores

- PiUpstreamClient: one Models per upstream holding only its provider; credential
  store adapter maps to that upstream's secret, so several accounts of one provider
  coexist; host env is only read for authKind none (ambient cloud creds)
- provider catalog from builtinProviders() plus static oauth modes, env fields and
  subscription terms notes; openai-compatible custom endpoints
- fake upstream provider with mock OAuth (browser paste / device approval)
- PiAgentRuntime accepts an upstream; thinking level clamped to the model

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/agent/src/index.ts

@@ -11,6 +11,14 @@ export {
 } from "./capability/registry.ts";
 export { capabilityIdToToolName, isCapabilityId, toolNameToCapabilityId } from "./capability/tool-name.ts";
 export * from "./capability/types.ts";
+export {
+  ENV_FIELDS,
+  OAUTH_CALLBACK_PORTS,
+  OAUTH_MODES,
+  PI_AI_VERSION,
+  providerCatalog,
+  providerInfo,
+} from "./pi/catalog.ts";
 export {
   type CreateFakeRuntimeOptions,
   createFakeRuntime,
@@ -22,6 +30,7 @@ export {
   type FakeToolCall,
   type FakeToolResult,
 } from "./pi/fake.ts";
+export { DEFAULT_FAKE_OAUTH, FakeOAuthControl } from "./pi/fake-upstream.ts";
 export {
   checkToolCall,
   DEFAULT_COMPACTION_INSTRUCTIONS,
@@ -29,6 +38,18 @@ export {
   type PiAgentRuntimeOptions,
 } from "./pi/runtime.ts";
 export { sessionPathFor } from "./pi/session.ts";
+export {
+  classifyUpstreamError,
+  PiUpstreamClient,
+  type UpstreamAuthEvent,
+  type UpstreamAuthPrompt,
+  type UpstreamClientOptions,
+  type UpstreamCredential,
+  type UpstreamCredentialStore,
+  UpstreamError,
+  type UpstreamLoginInteraction,
+  type UpstreamSpec,
+} from "./pi/upstream.ts";
 export { isSensitiveKey, REDACTED, redactForModel, redactString, summarizeForLog } from "./redact.ts";
 export * from "./runtime/port.ts";
 export {