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

packages/coding-agent/src/core/hooks/types.ts

@@ -426,7 +426,7 @@ export interface CommandContext {
 export interface RegisteredCommand {
 	name: string;
 	description?: string;
-	handler: (ctx: CommandContext) => Promise<string | undefined>;
+	handler: (ctx: CommandContext) => Promise<void>;
 }
 
 /**