Implement $(cmd) subcommand syntax in commands (issue #27)

Add $() for inline subcommand substitution that runs a command and
inserts its stdout (trailing newlines stripped) into the outer command
string. Supports nesting, and mixing with ${expr} and $NAME.

lcmd.c

@@ -1039,6 +1039,30 @@ int lushCmd_command (lua_State *L) {
 }
 
 
+/* ===== lushCmd_subcmd ===== */
+
+/*
+** Subcommand substitution: run a command, return stdout with trailing
+** newlines stripped.  Used for $(cmd) inside command strings.
+*/
+int lushCmd_subcmd (lua_State *L) {
+  size_t len;
+  const char *s;
+  /* run the command — pushes {code, stdout, stderr} table */
+  lushCmd_command(L);
+  /* extract .stdout from result table */
+  lua_getfield(L, -1, "stdout");
+  lua_remove(L, -2);  /* remove the table, keep stdout string */
+  /* strip trailing newlines */
+  s = lua_tolstring(L, -1, &len);
+  while (len > 0 && s[len - 1] == '\n')
+    len--;
+  lua_pushlstring(L, s, len);
+  lua_remove(L, -2);  /* remove original stdout string */
+  return 1;
+}
+
+
 /* ===== lushCmd_interactive ===== */
 
 int lushCmd_interactive (lua_State *L) {