fix(utils): guard os.fchmod for Windows in atomic_json_write

os.fchmod is Unix-only; the Windows os module has no fchmod (only
chmod). Passing mode= (e.g. 0o600 when saving the Hindsight config
during `hermes memory setup`) crashed on Windows with:

    AttributeError: module 'os' has no attribute 'fchmod'

Guard the fchmod fast-path with hasattr(os, "fchmod"). Skipping it on
Windows is safe: mkstemp already creates the temp file as 0o600, and
the existing post-replace os.chmod(real_path, mode) — already wrapped
in try/except — applies the final mode durably (as far as Windows
honors it).

Adds regression tests: one simulating a Windows os module without
fchmod (must not raise), and one asserting the durable 0o600 mode on
POSIX.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

utils.py

@@ -117,7 +117,10 @@ def atomic_json_write(
         suffix=".tmp",
     )
     try:
-        if mode is not None:
+        if mode is not None and hasattr(os, "fchmod"):
+            # fchmod is Unix-only; Windows' os module has no fchmod. Skipping it
+            # here is safe — mkstemp already created the temp file as 0o600, and
+            # the post-replace os.chmod below applies the final mode durably.
             os.fchmod(fd, mode)
         with os.fdopen(fd, "w", encoding="utf-8") as f:
             json.dump(