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/21-prompt-redesign.md

@@ -0,0 +1,27 @@
+# 21 — Prompt redesign
+
+**Status:** open
+
+Reconsider the prompt system. Some observations:
+
+## Functional prompts via metatables
+
+Functional prompts can be achieved in pure Lua without special support:
+
+```lua
+_PROMPT = setmetatable({}, { __tostring = function(self)
+    local user = os.getenv("USER") or os.getenv("USERNAME") or "user"
+    local cwd  = os.getenv("PWD") or io.popen("pwd"):read("*l") or "?"
+    return string.format("%s:%s$ ", user, cwd)
+end })
+```
+
+This suggests our bespoke prompt implementation (issue #09) may be overengineered. A template-based approach or just documenting the metatable trick might suffice.
+
+## Prompt2 depth
+
+Prompt2 is rigid — opening deeper and deeper blocks results in a prompt at the same level. Bash doesn't do depth indication either, so maybe this is fine. Options:
+
+- Pass depth into `_PROMPT2` somehow
+- Add indentation (spaces/tabs) after prompt2 on the C side
+- Accept it as-is (bash doesn't do this either)