Add SDK usage examples

12 examples showing increasing levels of customization:
- 01-minimal: all defaults
- 02-custom-model: model and thinking level
- 03-custom-prompt: replace or modify prompt
- 04-skills: discover, filter, merge skills
- 05-tools: built-in tools, custom tools
- 06-hooks: logging, blocking, result modification
- 07-context-files: AGENTS.md files
- 08-slash-commands: file-based commands
- 09-api-keys-and-oauth: API key resolution, OAuth config
- 10-settings: compaction, retry, terminal settings
- 11-sessions: persistence options
- 12-full-control: replace everything

Also exports FileSlashCommand type from index.ts

packages/coding-agent/examples/sdk/03-custom-prompt.ts

@@ -0,0 +1,44 @@
+/**
+ * Custom System Prompt
+ *
+ * Shows how to replace or modify the default system prompt.
+ */
+
+import { createAgentSession, SessionManager } from "../../src/index.js";
+
+// Option 1: Replace prompt entirely
+const { session: session1 } = await createAgentSession({
+	systemPrompt: `You are a helpful assistant that speaks like a pirate.
+Always end responses with "Arrr!"`,
+	sessionManager: SessionManager.inMemory(),
+});
+
+session1.subscribe((event) => {
+	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
+		process.stdout.write(event.assistantMessageEvent.delta);
+	}
+});
+
+console.log("=== Replace prompt ===");
+await session1.prompt("What is 2 + 2?");
+console.log("\n");
+
+// Option 2: Modify default prompt (receives default, returns modified)
+const { session: session2 } = await createAgentSession({
+	systemPrompt: (defaultPrompt) => `${defaultPrompt}
+
+## Additional Instructions
+- Always be concise
+- Use bullet points when listing things`,
+	sessionManager: SessionManager.inMemory(),
+});
+
+session2.subscribe((event) => {
+	if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
+		process.stdout.write(event.assistantMessageEvent.delta);
+	}
+});
+
+console.log("=== Modify prompt ===");
+await session2.prompt("List 3 benefits of TypeScript.");
+console.log();