Add issues #16–#23

#16 multiline shell commands
#17 _ scope (global vs local)
#18 path expansion (~)
#19 nested commands trigger prompt2
#20 simplify backtick return value
#21 prompt redesign
#22 implicit interactive commands (drop ! prefix)
#23 general cleanup and refactor

issues/19-nested-commands-prompt2.md

@@ -0,0 +1,12 @@
+# 19 — Nested commands in interactive mode trigger prompt2
+
+**Status:** open
+
+Running nested commands in an interactive command drops into prompt2 when it shouldn't:
+
+```
+~/Code/20251000_lush> !echo ${`ls`.stdout}
+>>
+```
+
+The parser sees the opening `{` and thinks the expression is incomplete, waiting for more input. The nested backtick command should be parsed and evaluated inline.