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/17-underscore-scope.md

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+# 17 — `_` is global rather than local
+
+**Status:** open
+
+The `_` result variable from interactive commands (`!cmd`) is set as a global via `lua_setglobal(L, "_")`. This has several implications:
+
+- It can escape its scope and pollute other scopes
+- A function may silently overwrite `_` if it uses shell commands internally, surprising the caller
+- Lua convention uses `_` as a throwaway variable in `for` loops (`for _, v in pairs(t)`)
+
+## Investigation needed
+
+- What are the concrete consequences of `_` being global vs local?
+- Can we make it local to the REPL line instead?
+- Should we use a different name entirely?