Plugins: remove public extension-api surface (#48462)

* Plugins: remove public extension-api surface

* Plugins: fix loader setup routing follow-ups

* CI: ignore non-extension helper dirs in extension-fast

* Docs: note extension-api removal as breaking

src/plugins/runtime/runtime-agent.ts

@@ -0,0 +1,36 @@
+import { resolveAgentDir, resolveAgentWorkspaceDir } from "../../agents/agent-scope.js";
+import { DEFAULT_MODEL, DEFAULT_PROVIDER } from "../../agents/defaults.js";
+import { resolveAgentIdentity } from "../../agents/identity.js";
+import { resolveThinkingDefault } from "../../agents/model-selection.js";
+import { runEmbeddedPiAgent } from "../../agents/pi-embedded.js";
+import { resolveAgentTimeoutMs } from "../../agents/timeout.js";
+import { ensureAgentWorkspace } from "../../agents/workspace.js";
+import {
+  loadSessionStore,
+  resolveSessionFilePath,
+  resolveStorePath,
+  saveSessionStore,
+} from "../../config/sessions.js";
+import type { PluginRuntime } from "./types.js";
+
+export function createRuntimeAgent(): PluginRuntime["agent"] {
+  return {
+    defaults: {
+      model: DEFAULT_MODEL,
+      provider: DEFAULT_PROVIDER,
+    },
+    resolveAgentDir,
+    resolveAgentWorkspaceDir,
+    resolveAgentIdentity,
+    resolveThinkingDefault,
+    runEmbeddedPiAgent,
+    resolveAgentTimeoutMs,
+    ensureAgentWorkspace,
+    session: {
+      resolveStorePath,
+      loadSessionStore,
+      saveSessionStore,
+      resolveSessionFilePath,
+    },
+  };
+}