Add handoff example hook

Transfers context to a new focused session instead of compacting.
User provides a goal, hook generates a prompt with relevant context,
creates a new session with parent tracking, and loads the prompt
as a draft in the editor for review.

Inspired by Amp's handoff feature.

packages/coding-agent/examples/hooks/handoff.ts

@@ -0,0 +1,159 @@
+/**
+ * Handoff hook - transfer context to a new focused session
+ *
+ * Instead of compacting (which is lossy), handoff extracts what matters
+ * for your next task and creates a new session with a generated prompt.
+ *
+ * Usage:
+ *   /handoff now implement this for teams as well
+ *   /handoff execute phase one of the plan
+ *   /handoff check other places that need this fix
+ *
+ * The generated prompt appears as a draft in the editor for review/editing.
+ */
+
+import { complete, type Message } from "@mariozechner/pi-ai";
+import type { HookAPI } from "@mariozechner/pi-coding-agent";
+import { BorderedLoader } from "@mariozechner/pi-coding-agent";
+
+const SYSTEM_PROMPT = `You are a context transfer assistant. Given a conversation history and the user's goal for a new thread, generate a focused prompt that:
+
+1. Summarizes relevant context from the conversation (decisions made, approaches taken, key findings)
+2. Lists any relevant files that were discussed or modified
+3. Clearly states the next task based on the user's goal
+4. Is self-contained - the new thread should be able to proceed without the old conversation
+
+Format your response as a prompt the user can send to start the new thread. Be concise but include all necessary context. Do not include any preamble like "Here's the prompt" - just output the prompt itself.
+
+Example output format:
+## Context
+We've been working on X. Key decisions:
+- Decision 1
+- Decision 2
+
+Files involved:
+- path/to/file1.ts
+- path/to/file2.ts
+
+## Task
+[Clear description of what to do next based on user's goal]`;
+
+export default function (pi: HookAPI) {
+	pi.registerCommand("handoff", {
+		description: "Transfer context to a new focused session",
+		handler: async (args, ctx) => {
+			if (!ctx.hasUI) {
+				ctx.ui.notify("handoff requires interactive mode", "error");
+				return;
+			}
+
+			if (!ctx.model) {
+				ctx.ui.notify("No model selected", "error");
+				return;
+			}
+
+			const goal = args.trim();
+			if (!goal) {
+				ctx.ui.notify("Usage: /handoff <goal for new thread>", "error");
+				return;
+			}
+
+			// Gather conversation context from current branch
+			const branch = ctx.sessionManager.getBranch();
+			const conversationParts: string[] = [];
+
+			for (const entry of branch) {
+				if (entry.type === "message") {
+					const msg = entry.message;
+					if ("role" in msg && (msg.role === "user" || msg.role === "assistant")) {
+						let text: string;
+						if (typeof msg.content === "string") {
+							text = msg.content;
+						} else {
+							text = msg.content
+								.filter((c): c is { type: "text"; text: string } => c.type === "text")
+								.map((c) => c.text)
+								.join("\n");
+						}
+						if (text) {
+							const role = msg.role === "user" ? "User" : "Assistant";
+							conversationParts.push(`${role}: ${text}`);
+						}
+					}
+				}
+			}
+
+			if (conversationParts.length === 0) {
+				ctx.ui.notify("No conversation to hand off", "error");
+				return;
+			}
+
+			const conversationText = conversationParts.join("\n\n");
+			const currentSessionFile = ctx.sessionManager.getSessionFile();
+
+			// Generate the handoff prompt with loader UI
+			const result = await ctx.ui.custom<string | null>((tui, theme, done) => {
+				const loader = new BorderedLoader(tui, theme, `Generating handoff prompt...`);
+				loader.onAbort = () => done(null);
+
+				const doGenerate = async () => {
+					const apiKey = await ctx.modelRegistry.getApiKey(ctx.model!);
+
+					const userMessage: Message = {
+						role: "user",
+						content: [
+							{
+								type: "text",
+								text: `## Conversation History\n\n${conversationText}\n\n## User's Goal for New Thread\n\n${goal}`,
+							},
+						],
+						timestamp: Date.now(),
+					};
+
+					const response = await complete(
+						ctx.model!,
+						{ systemPrompt: SYSTEM_PROMPT, messages: [userMessage] },
+						{ apiKey, signal: loader.signal },
+					);
+
+					if (response.stopReason === "aborted") {
+						return null;
+					}
+
+					return response.content
+						.filter((c): c is { type: "text"; text: string } => c.type === "text")
+						.map((c) => c.text)
+						.join("\n");
+				};
+
+				doGenerate()
+					.then(done)
+					.catch((err) => {
+						console.error("Handoff generation failed:", err);
+						done(null);
+					});
+
+				return loader;
+			});
+
+			if (result === null) {
+				ctx.ui.notify("Cancelled", "info");
+				return;
+			}
+
+			// Create new session with parent tracking
+			const newSessionResult = await ctx.newSession({
+				parentSession: currentSessionFile,
+			});
+
+			if (newSessionResult.cancelled) {
+				ctx.ui.notify("New session cancelled", "info");
+				return;
+			}
+
+			// Set the generated prompt as a draft in the editor
+			ctx.ui.setEditorText(result);
+			ctx.ui.notify("Handoff ready. Review the prompt and submit when ready.", "info");
+		},
+	});
+}