Implement prefix-based interactive commands via ! (issue #14)

Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.

issues/14-prefix-interactive-commands.md

@@ -1,6 +1,6 @@
 # Issue #14 — Prefix-based interactive command execution
 
-**Status:** open
+**Status:** done
 **Alternative to:** #10, #12
 
 ## Motivation