fix(codex): BSD mktemp templates broke /codex on every macOS install (#2091)

macOS mktemp requires the X's to end the template; the five
"codex-*-XXXXXX.txt" sites failed with "mkstemp failed ... File exists"
before Codex ever ran (reproduced live on this machine). Same class fixed
in claude/SKILL.md.tmpl's three sites. bin/gstack-paths now strips macOS's
trailing slash from TMPDIR so TMP_ROOT-built paths stop carrying "//".

Static tripwire scans every tracked .tmpl for characters after the X-run in
a mktemp template (longer X-runs stay valid), plus a live portability check
of the emitted shape.

Co-authored-by: Sina Matian <sina@time-attack.dev>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/mktemp-portability.test.ts

@@ -0,0 +1,57 @@
+/**
+ * Regression pin for #2091: /codex was broken on every macOS install because
+ * its mktemp templates carried a suffix after the X's ("codex-err-XXXXXX.txt").
+ * BSD mktemp (macOS) requires the X's to be the trailing characters of the
+ * template; with a suffix it fails ("mkstemp failed ... File exists"), the
+ * temp file never exists, and the skill dies before Codex ever runs.
+ *
+ * GNU mktemp accepts a --suffix flag but ALSO rejects inline suffixes in the
+ * template argument on BusyBox, so the portable form is: X's last, no suffix.
+ *
+ * This scans every .tmpl (the sources of truth — generated SKILL.md files
+ * follow at regen time) for the broken shape.
+ */
+
+import { describe, it, expect } from "bun:test";
+import { execFileSync } from "child_process";
+import { readFileSync } from "fs";
+import { join } from "path";
+
+const ROOT = join(import.meta.dir, "..");
+
+function trackedTmplFiles(): string[] {
+  const out = execFileSync("git", ["ls-files", "*.tmpl", "**/*.tmpl"], {
+    cwd: ROOT,
+    encoding: "utf-8",
+  });
+  return out.split("\n").filter(Boolean);
+}
+
+describe("mktemp portability (#2091)", () => {
+  it("no .tmpl file uses a mktemp template with characters after XXXXXX", () => {
+    const offenders: string[] = [];
+    for (const rel of trackedTmplFiles()) {
+      const src = readFileSync(join(ROOT, rel), "utf-8");
+      src.split("\n").forEach((line, i) => {
+        // Broken shape: the X-run followed by a non-quote, non-whitespace,
+        // non-closing character inside a mktemp invocation. X{6,} is greedy,
+        // so longer X-runs (spec's XXXXXXXX) stay valid — only a genuine
+        // suffix after the final X trips it.
+        if (/mktemp[^\n]*X{6,}[^"'\s)X]/.test(line)) {
+          offenders.push(`${rel}:${i + 1}: ${line.trim()}`);
+        }
+      });
+    }
+    expect(offenders).toEqual([]);
+  });
+
+  it("BSD-portable form actually works on this platform", () => {
+    // Live sanity: the exact template shape the skills now emit.
+    const tmp = process.env.TMPDIR || "/tmp";
+    const created = execFileSync("mktemp", [`${tmp.replace(/\/$/, "")}/gstack-portability-XXXXXX`], {
+      encoding: "utf-8",
+    }).trim();
+    expect(created.length).toBeGreaterThan(0);
+    execFileSync("rm", ["-f", created]);
+  });
+});