Remove the open command

`open` and `view` shared one implementation and differed in a single
line: `open` also made the project the active one. Only `view` is left,
with the behaviour it always had.

Dropping it from builtinCmds is the part worth noting: a reserved word
cannot be shadowed by an alias, so `open` is now free for one --
`alias open '!xdg-open $1'` works, which it could not before. That also
made a completion test wrong, since it used `open` as its example of a
name a builtin owns; it uses `status` now.

The `runInDir(d, "open", ...)` calls stay: those are macOS's open(1),
which is how an Xcode workspace gets opened.

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

alias.go

@@ -46,7 +46,7 @@ var builtinCmds = map[string]bool{
 	"diff": true, "pull": true, "fetch": true, "push": true, "edit": true,
 	"pushremote": true, "overview": true, "archive": true, "init": true,
 	"login": true, "cd": true, "checkout": true, "clone": true, "cloneall": true,
-	"open": true, "view": true, "count": true, "tag": true, "alias": true,
+	"view": true, "count": true, "tag": true, "alias": true,
 	"unalias": true, "config": true, "release": true,
 }