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>

completion.go

@@ -45,14 +45,13 @@ func runeSuffixes(cands []string, prefix string) ([][]rune, int) {
 }
 
 // builtinCompleter is the command tree. Command names complete at the start of
-// the line; cd/open/view complete local project names; clone/show complete
+// the line; cd/view complete local project names; clone/show complete
 // repository names cached from the git server; checkout/tag complete branch and
 // tag names; pushremote/release complete mirror targets; dist completes
 // filesystem paths.
 func builtinCompleter() *readline.PrefixCompleter {
 	return readline.NewPrefixCompleter(
 		readline.PcItem("cd", readline.PcItemDynamic(dynLocalProjects)),
-		readline.PcItem("open", readline.PcItemDynamic(dynLocalProjects)),
 		readline.PcItem("view", readline.PcItemDynamic(dynLocalProjects)),
 		readline.PcItem("clone",
 			readline.PcItem("-a", readline.PcItemDynamic(dynServerArchives)),