feat(setup): add Codex CLI setup recipe (#1243)

* Add Codex setup recipe

* Sync beads issues (bd-1zo)

---------

Co-authored-by: Amp <amp@example.com>

cmd/bd/setup/codex_test.go

@@ -0,0 +1,36 @@
+package setup
+
+import (
+	"strings"
+	"testing"
+)
+
+func stubCodexEnvProvider(t *testing.T, env agentsEnv) {
+	t.Helper()
+	orig := codexEnvProvider
+	codexEnvProvider = func() agentsEnv {
+		return env
+	}
+	t.Cleanup(func() { codexEnvProvider = orig })
+}
+
+func TestInstallCodexCreatesNewFile(t *testing.T) {
+	env, stdout, _ := newFactoryTestEnv(t)
+	if err := installCodex(env); err != nil {
+		t.Fatalf("installCodex returned error: %v", err)
+	}
+	if !strings.Contains(stdout.String(), "Codex CLI integration installed") {
+		t.Error("expected Codex install success message")
+	}
+}
+
+func TestCheckCodexMissingFile(t *testing.T) {
+	env, stdout, _ := newFactoryTestEnv(t)
+	err := checkCodex(env)
+	if err == nil {
+		t.Fatal("expected error for missing AGENTS.md")
+	}
+	if !strings.Contains(stdout.String(), "bd setup codex") {
+		t.Error("expected setup guidance for codex")
+	}
+}