add jjcommit script

$diff_output"

bin/jjcommit.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+set -e
+
+# Run jj diff and prefix each line with "JJ: "
+diff_output="$(jj diff --git | sed 's/^/JJ: /')"
+
+# Add two newlines at the top (without the prefix)
+final_output="\n\n$diff_output"
+
+# Feed the string to jj desc --stdin
+# Use printf to preserve newlines
+printf "%b" "$final_output" | jj desc --stdin --edit