Implement $VAR expansion in commands (issue #25)

Extend read_command_body() to detect $NAME and trigger interpolation
using the same fragment-split mechanism as ${expr}. The lexer collects
the identifier into cmd_envvar; the parser's unified parseinterp()
branches on it to emit tostring(getenv(NAME)).

issues/25-envvar-expansion-in-commands.md

@@ -1,6 +1,6 @@
 # 25 — Environment variable expansion in commands
 
-**Status:** open
+**Status:** done
 
 ## Problem