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>

config.go

@@ -33,7 +33,7 @@ type Config struct {
 	GitName     string // git user.name to set globally ("" = leave alone)
 	GitEmail    string // git user.email to set globally ("" = leave alone)
 	PushDefault string // git push.default to set globally ("" = leave alone)
-	Editor      string // editor/opener used as fallback by `open` ("" = coda)
+	Editor      string // editor/opener used as fallback by `view` ("" = coda)
 	Mirror      string // truthy -> `push` also mirrors via `pushremote`
 	SecretScan  string // falsy -> `push` skips the credential scan
 	Remotes     []RemoteTarget