feat: add basic prompt submit hook (#694)

hooks/prompt-instructions.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+# UserPromptSubmit hook - adds instructions to every prompt
+# Reads JSON from stdin, outputs instructions to stdout, exits 0
+
+# Consume stdin (required for hook protocol)
+cat > /dev/null
+
+# Output instructions that will be injected into agent context
+echo "Be specific. Double check your work. Ask clarifying questions."
+
+exit 0