Implement programmable prompt (issue #09)

Replace static _PROMPT/_PROMPT2 with dynamic __prompt(exitcode)/__prompt2(exitcode)
functions. Fallback chain: __prompt() → _PROMPT → "> ". Install a default __prompt
that shows the current directory with ~ abbreviation. Track last exit code from the
REPL loop and pass it to the prompt function.

issues/09-prompt.md

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