test(ninja): plugin tags subcommand round-trip

Adds TestMutateTags_AddRmSetClear (covers add/rm/set/clear operations
including dedupe and normalisation) and TestMutateTags_RejectsInvalidNoWrite
(ensures validation failures don't mutate plugin.mod).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/ninja/cmd/plugin_test.go

@@ -548,6 +548,78 @@ func TestParsePrivateCoord(t *testing.T) {
 	}
 }
 
+func TestMutateTags_AddRmSetClear(t *testing.T) {
+	dir := t.TempDir()
+	t.Chdir(dir)
+
+	must := func(err error) {
+		t.Helper()
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// Seed plugin.mod with no tags.
+	must(upsertPluginMod("themes", "darkpro", "Dark Pro", "Sleek dark theme", "theme", []string{}, nil, false))
+
+	// add
+	must(mutateTags("add", []string{"dark", "agency"}))
+	mod, err := readLocalMod()
+	must(err)
+	if len(mod.Plugin.Tags) != 2 || mod.Plugin.Tags[0] != "dark" || mod.Plugin.Tags[1] != "agency" {
+		t.Errorf("after add: %v", mod.Plugin.Tags)
+	}
+
+	// add (dedupe + normalise)
+	must(mutateTags("add", []string{"Agency", "Serif"}))
+	mod, _ = readLocalMod()
+	if len(mod.Plugin.Tags) != 3 {
+		t.Errorf("after dedupe add: %v", mod.Plugin.Tags)
+	}
+
+	// rm
+	must(mutateTags("rm", []string{"dark"}))
+	mod, _ = readLocalMod()
+	for _, tag := range mod.Plugin.Tags {
+		if tag == "dark" {
+			t.Errorf("after rm: dark still present: %v", mod.Plugin.Tags)
+		}
+	}
+
+	// set
+	must(mutateTags("set", []string{"editorial"}))
+	mod, _ = readLocalMod()
+	if len(mod.Plugin.Tags) != 1 || mod.Plugin.Tags[0] != "editorial" {
+		t.Errorf("after set: %v", mod.Plugin.Tags)
+	}
+
+	// clear
+	must(mutateTags("clear", nil))
+	mod, _ = readLocalMod()
+	if len(mod.Plugin.Tags) != 0 {
+		t.Errorf("after clear: %v", mod.Plugin.Tags)
+	}
+}
+
+func TestMutateTags_RejectsInvalidNoWrite(t *testing.T) {
+	dir := t.TempDir()
+	t.Chdir(dir)
+
+	if err := upsertPluginMod("themes", "x", "X", "", "theme", nil, []string{"dark"}, false); err != nil {
+		t.Fatal(err)
+	}
+	if err := mutateTags("add", []string{"BAD SPACE"}); err == nil {
+		t.Fatal("expected validation error")
+	}
+	mod, err := readLocalMod()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(mod.Plugin.Tags) != 1 || mod.Plugin.Tags[0] != "dark" {
+		t.Errorf("tags mutated despite error: %v", mod.Plugin.Tags)
+	}
+}
+
 func runGit(t *testing.T, dir string, args ...string) {
 	t.Helper()
 	cmd := exec.Command("git", args...)