Hook commands: remove string return, use sendMessage() for prompting

- Command handler now returns Promise<void> instead of Promise<string | undefined>
- To trigger LLM response, use sendMessage() with triggerTurn: true
- Simplify _tryExecuteHookCommand to return boolean

Added example hook and slash command in .pi/:
- .pi/hooks/test-command.ts - /greet command using sendMessage
- .pi/commands/review.md - file-based /review command

.pi/hooks/test-command.ts

@@ -0,0 +1,24 @@
+/**
+ * Test hook that registers a /greet command.
+ * Usage: /greet [name]
+ */
+import type { HookAPI } from "@mariozechner/pi-coding-agent/hooks";
+
+export default function (pi: HookAPI) {
+	pi.registerCommand("greet", {
+		description: "Send a greeting message to the LLM",
+		handler: async (ctx) => {
+			const name = ctx.args.trim() || "world";
+
+			// Insert a custom message and trigger LLM response
+			ctx.sendMessage(
+				{
+					customType: "greeting",
+					content: `Hello, ${name}! Please say something nice about them.`,
+					display: true,
+				},
+				true, // triggerTurn - get LLM to respond
+			);
+		},
+	});
+}