feat: Expose BuildSystemPromptOptions on before-agent-start event. (#3473)

Resolves: #3463

packages/coding-agent/test/agent-session-concurrent.test.ts

@@ -21,6 +21,7 @@ import { AuthStorage } from "../src/core/auth-storage.js";
 import { ModelRegistry } from "../src/core/model-registry.js";
 import { SessionManager } from "../src/core/session-manager.js";
 import { SettingsManager } from "../src/core/settings-manager.js";
+import type { BuildSystemPromptOptions } from "../src/core/system-prompt.js";
 import { createTestExtensionsResult, createTestResourceLoader } from "./utilities.js";
 
 // Mock stream that mimics AssistantMessageEventStream
@@ -443,7 +444,12 @@ describe("AgentSession concurrent prompt guard", () => {
 					images: unknown,
 					source: "interactive" | "rpc" | "extension",
 				) => Promise<{ action: "continue" }>;
-				emitBeforeAgentStart: (prompt: string, images: unknown, systemPrompt: string) => Promise<undefined>;
+				emitBeforeAgentStart: (
+					prompt: string,
+					images: unknown,
+					systemPrompt: string,
+					systemPromptOptions: BuildSystemPromptOptions,
+				) => Promise<undefined>;
 			};
 		};
 		sessionWithRunner._extensionRunner = {
@@ -578,7 +584,12 @@ describe("AgentSession concurrent prompt guard", () => {
 					images: unknown,
 					source: "interactive" | "rpc" | "extension",
 				) => Promise<{ action: "continue" }>;
-				emitBeforeAgentStart: (prompt: string, images: unknown, systemPrompt: string) => Promise<undefined>;
+				emitBeforeAgentStart: (
+					prompt: string,
+					images: unknown,
+					systemPrompt: string,
+					systemPromptOptions: BuildSystemPromptOptions,
+				) => Promise<undefined>;
 			};
 		};
 		sessionWithRunner._extensionRunner = {