fix(redact): tolerate EEXIST from recursive mkdir in install-prepush-hook on bun/Windows (#2635)

fs.mkdirSync(dir, { recursive: true }) is a no-op on an existing directory
in Node, but bun on Windows throws EEXIST - crashing hook install on any
repo whose .git/hooks already existed, leaving the repo unprotected.

Add lib/fs-utils.ts mkdirpSync: swallow EEXIST only when statSync confirms
the path is an existing directory; a regular file occupying the path, a
stat failure, or any other errno still rethrows. Use it in
installPrepushHook().

The regression test emulates the Windows bun fs semantics via a
bun --preload fixture, so the exact crash path runs (and fails on the old
code) on any platform, including CI Linux.

Absorbed from PR #2641 with authorship preserved.

Fixes #2635

test/helpers/emulate-bun-windows-eexist.ts

@@ -0,0 +1,20 @@
+/**
+ * Bun --preload fixture that emulates bun-on-Windows fs.mkdirSync semantics
+ * (see #2635): a recursive mkdir on an already-existing directory throws
+ * EEXIST, where Node (and bun on Linux/macOS) treat it as a no-op success.
+ *
+ * Loaded into a child process with `bun --preload <this file> <script>`, it
+ * lets the #2635 regression test exercise the exact Windows crash path on any
+ * platform. The patch is deliberately transparent - it changes nothing except
+ * throwing EEXIST where Windows bun would.
+ */
+const fs = require("fs");
+const orig = fs.mkdirSync;
+fs.mkdirSync = (p: string, opts: any) => {
+  if (opts?.recursive && fs.existsSync(p) && fs.statSync(p).isDirectory()) {
+    const e = new Error(`EEXIST: file already exists, mkdir '${p}'`);
+    (e as any).code = "EEXIST";
+    throw e;
+  }
+  return orig(p, opts);
+};