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/commands/review.md

@@ -0,0 +1,12 @@
+---
+description: Review a file for issues
+---
+Please review the following file for potential issues, bugs, or improvements:
+
+$1
+
+Focus on:
+- Logic errors
+- Edge cases
+- Code style
+- Performance concerns