Redesign prompt system: revert #09, use standard _PROMPT (issue #21)

Revert the bespoke __prompt()/__prompt2() function mechanism from issue
#09 and return to stock Lua's _PROMPT/_PROMPT2 globals (which already
support dynamic prompts via __tostring metatables). Set default
_PROMPT = "lush> " in doREPL if not already defined by config.

issues/09-prompt.md

@@ -1,6 +1,7 @@
 # 09 — Programmable prompt
 
-**Status:** done
+**Status:** open
+**Blocked by:** #08
 
 Users should be able to customize the shell prompt by defining a Lua function, similar to how other shells use `PS1`/`PROMPT_COMMAND`/`precmd`.
 

issues/21-prompt-redesign.md

@@ -1,6 +1,6 @@
 # 21 — Prompt redesign
 
-**Status:** open
+**Status:** done
 
 Reconsider the prompt system. Some observations: