Complete aliases that expand to a shell escape

`alias ll '!ls -la'` makes everything after `ll` a shell argument just as
surely as typing the `!` does, but Tab there still went to the builtin
command tree and found nothing. The dispatch now asks what a line will
turn into rather than how it starts: a '!' escape, or a name that is not a
builtin and resolves to an alias whose body starts with '!'.

Only the arguments complete — the command word is fixed by the alias
body, so `ll vi` offers the file, never the editor. An alias to a builtin
stays with the builtin tree.

Only the alias itself is inspected, not what its expansion might expand
to in turn: an alias chain can rewrite its own arguments, and guessing at
that would offer candidates for a command line other than the one being
built.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

completion.go

@@ -9,9 +9,10 @@ import (
 	"github.com/chzyer/readline"
 )
 
-// completer wires up Tab completion. A line starting with '!' is completed the
-// way a shell would — executables for the command, paths for its arguments —
-// and everything else against the builtin command tree.
+// completer wires up Tab completion. A line headed for a shell — a '!' escape,
+// or an alias that expands to one — is completed the way a shell would:
+// executables for the command, paths for its arguments. Everything else goes to
+// the builtin command tree.
 func completer() readline.AutoCompleter {
 	return &mgshCompleter{builtin: builtinCompleter()}
 }
@@ -23,8 +24,7 @@ func (c *mgshCompleter) Do(line []rune, pos int) ([][]rune, int) {
 	if pos > len(line) {
 		pos = len(line)
 	}
-	if strings.HasPrefix(strings.TrimLeft(string(line[:pos]), " \t"), "!") {
-		cands, prefix := shellCandidates(string(line[:pos]))
+	if cands, prefix, ok := completeShellLine(string(line[:pos])); ok {
 		return runeSuffixes(cands, prefix)
 	}
 	return c.builtin.Do(line, pos)