Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/examples/extensions/qna.ts

@@ -0,0 +1,119 @@
+/**
+ * Q&A extraction extension - extracts questions from assistant responses
+ *
+ * Demonstrates the "prompt generator" pattern:
+ * 1. /qna command gets the last assistant message
+ * 2. Shows a spinner while extracting (hides editor)
+ * 3. Loads the result into the editor for user to fill in answers
+ */
+
+import { complete, type UserMessage } from "@mariozechner/pi-ai";
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+import { BorderedLoader } from "@mariozechner/pi-coding-agent";
+
+const SYSTEM_PROMPT = `You are a question extractor. Given text from a conversation, extract any questions that need answering and format them for the user to fill in.
+
+Output format:
+- List each question on its own line, prefixed with "Q: "
+- After each question, add a blank line for the answer prefixed with "A: "
+- If no questions are found, output "No questions found in the last message."
+
+Example output:
+Q: What is your preferred database?
+A: 
+
+Q: Should we use TypeScript or JavaScript?
+A: 
+
+Keep questions in the order they appeared. Be concise.`;
+
+export default function (pi: ExtensionAPI) {
+	pi.registerCommand("qna", {
+		description: "Extract questions from last assistant message into editor",
+		handler: async (_args, ctx) => {
+			if (!ctx.hasUI) {
+				ctx.ui.notify("qna requires interactive mode", "error");
+				return;
+			}
+
+			if (!ctx.model) {
+				ctx.ui.notify("No model selected", "error");
+				return;
+			}
+
+			// Find the last assistant message on the current branch
+			const branch = ctx.sessionManager.getBranch();
+			let lastAssistantText: string | undefined;
+
+			for (let i = branch.length - 1; i >= 0; i--) {
+				const entry = branch[i];
+				if (entry.type === "message") {
+					const msg = entry.message;
+					if ("role" in msg && msg.role === "assistant") {
+						if (msg.stopReason !== "stop") {
+							ctx.ui.notify(`Last assistant message incomplete (${msg.stopReason})`, "error");
+							return;
+						}
+						const textParts = msg.content
+							.filter((c): c is { type: "text"; text: string } => c.type === "text")
+							.map((c) => c.text);
+						if (textParts.length > 0) {
+							lastAssistantText = textParts.join("\n");
+							break;
+						}
+					}
+				}
+			}
+
+			if (!lastAssistantText) {
+				ctx.ui.notify("No assistant messages found", "error");
+				return;
+			}
+
+			// Run extraction with loader UI
+			const result = await ctx.ui.custom<string | null>((tui, theme, done) => {
+				const loader = new BorderedLoader(tui, theme, `Extracting questions using ${ctx.model!.id}...`);
+				loader.onAbort = () => done(null);
+
+				// Do the work
+				const doExtract = async () => {
+					const apiKey = await ctx.modelRegistry.getApiKey(ctx.model!);
+					const userMessage: UserMessage = {
+						role: "user",
+						content: [{ type: "text", text: lastAssistantText! }],
+						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");
+				};
+
+				doExtract()
+					.then(done)
+					.catch(() => done(null));
+
+				return loader;
+			});
+
+			if (result === null) {
+				ctx.ui.notify("Cancelled", "info");
+				return;
+			}
+
+			ctx.ui.setEditorText(result);
+			ctx.ui.notify("Questions loaded. Edit and submit when ready.", "info");
+		},
+	});
+}