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>

shellcomplete_test.go

@@ -214,10 +214,10 @@ func TestShellCandidatesThroughAlias(t *testing.T) {
 	shellTree(t)
 	fakeCommands(t, "vi", "ls")
 	withAliases(t, map[string]string{
-		"ll":   "!ls -la",
-		"e":    "!vi $1",
-		"co":   "checkout $1",  // a builtin, not a shell command
-		"open": "!xdg-open $1", // shadows a builtin: must not count
+		"ll":     "!ls -la",
+		"e":      "!vi $1",
+		"co":     "checkout $1", // expands to a builtin, not a shell command
+		"status": "!git status", // shadows a builtin: must not count
 	})
 
 	// arguments of a shell alias complete against the filesystem
@@ -246,7 +246,7 @@ func TestShellCandidatesThroughAlias(t *testing.T) {
 		t.Error("an alias expanding to a builtin was treated as a shell line")
 	}
 	// nor is a name that a builtin owns, since runCommand never expands those
-	if _, _, ok := shellLine("open ma"); ok {
+	if _, _, ok := shellLine("status ma"); ok {
 		t.Error("a builtin name was resolved through an alias")
 	}
 	// nor an undefined name