feat(agent): AgentHarness streams through a required Models instance (phase 6)

AgentHarnessOptions.models is required; the harness stream path,
compaction, and branch summarization go through models.streamSimple()/
completeSimple() instead of the compat globals. getApiKeyAndHeaders
stays and wins per-field over provider-resolved auth, but is no longer
required: without it, requests resolve through provider auth.

compact()/generateSummary()/generateBranchSummary() take a Models
parameter; explicit apiKey becomes optional. StreamFn is redefined
structurally (Models.streamSimple satisfies it), dropping the compat
type dependency from agent types.

Harness tests build per-file Models collections with fauxProvider()
and unique provider ids instead of mutating the global api-registry.

packages/agent/test/scratch/simple.ts

@@ -1,6 +1,8 @@
 import { homedir } from "node:os";
 import { join } from "node:path";
-import { getModel } from "@earendil-works/pi-ai/compat";
+import { createModels } from "@earendil-works/pi-ai";
+import { getBuiltinModel } from "@earendil-works/pi-ai/providers/all";
+import { openaiProvider } from "@earendil-works/pi-ai/providers/openai";
 import { NodeExecutionEnv } from "../../src/harness/env/nodejs.ts";
 import { InMemorySessionStorage } from "../../src/harness/session/memory-storage.ts";
 import {
@@ -35,11 +37,15 @@ const { promptTemplates: sourcedPromptTemplates } = await loadSourcedPromptTempl
 	(promptTemplate, source) => ({ ...promptTemplate, source }),
 );
 
+const models = createModels();
+models.setProvider(openaiProvider());
+
 const session = new Session(new InMemorySessionStorage());
 const agent = new AgentHarness({
 	env,
 	session,
-	model: getModel("openai", "gpt-5.5"),
+	models,
+	model: getBuiltinModel("openai", "gpt-5.5"),
 	thinkingLevel: "low",
 	systemPrompt: ({ env, resources }) =>
 		[