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/confirm-destructive.ts

@@ -0,0 +1,59 @@
+/**
+ * Confirm Destructive Actions Extension
+ *
+ * Prompts for confirmation before destructive session actions (clear, switch, branch).
+ * Demonstrates how to cancel session events using the before_* events.
+ */
+
+import type { ExtensionAPI, SessionBeforeSwitchEvent, SessionMessageEntry } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("session_before_switch", async (event: SessionBeforeSwitchEvent, ctx) => {
+		if (!ctx.hasUI) return;
+
+		if (event.reason === "new") {
+			const confirmed = await ctx.ui.confirm(
+				"Clear session?",
+				"This will delete all messages in the current session.",
+			);
+
+			if (!confirmed) {
+				ctx.ui.notify("Clear cancelled", "info");
+				return { cancel: true };
+			}
+			return;
+		}
+
+		// reason === "resume" - check if there are unsaved changes (messages since last assistant response)
+		const entries = ctx.sessionManager.getEntries();
+		const hasUnsavedWork = entries.some(
+			(e): e is SessionMessageEntry => e.type === "message" && e.message.role === "user",
+		);
+
+		if (hasUnsavedWork) {
+			const confirmed = await ctx.ui.confirm(
+				"Switch session?",
+				"You have messages in the current session. Switch anyway?",
+			);
+
+			if (!confirmed) {
+				ctx.ui.notify("Switch cancelled", "info");
+				return { cancel: true };
+			}
+		}
+	});
+
+	pi.on("session_before_branch", async (event, ctx) => {
+		if (!ctx.hasUI) return;
+
+		const choice = await ctx.ui.select(`Branch from entry ${event.entryId.slice(0, 8)}?`, [
+			"Yes, create branch",
+			"No, stay in current session",
+		]);
+
+		if (choice !== "Yes, create branch") {
+			ctx.ui.notify("Branch cancelled", "info");
+			return { cancel: true };
+		}
+	});
+}