fix(secure-state): skip directory fsync on Windows (unsupported, blocks install)

writePrivateJson fsyncs the parent directory handle after the atomic rename.
Windows does not support fsync on a directory handle — FlushFileBuffers on a
directory handle returns ERROR_ACCESS_DENIED, which Node surfaces as
`EPERM: operation not permitted, fsync`. This throws on the very first key
write, so `agent-config-init.mjs` / `murmur-join.mjs` cannot complete and
Murmur cannot be installed on Windows at all.

The key file itself is already fsync'd above the rename; the directory fsync is
a best-effort durability nicety. Guard it behind `process.platform !== 'win32'`:
POSIX behaviour is byte-identical, Windows skips only the unsupported call.

Verified on Windows 11 / Node 24.13.1: `murmur-join.mjs` completes and writes
agent-config.json where it previously threw EPERM at secure-state.mjs:90.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>