fix(ssh): handle WinError 1314 symlink failure with shutil.copy2 fallback

On Windows, os.symlink() raises OSError (WinError 1314) unless the
process has Administrator rights or Developer Mode is enabled. The SSH
bulk-upload staging logic used symlinks to mirror the remote layout
before piping through tar; this caused all ssh_bulk_upload tests to
fail on Windows.

- ssh.py: wrap os.symlink() in try/except OSError and fall back to
  shutil.copy2() so staging works on every platform. shutil was already
  imported, no new dependency introduced.
- file_sync.py: replace str(Path(remote).parent) with
  posixpath.dirname(remote) in unique_parent_dirs(). pathlib.Path uses
  the host separator (\ on Windows), but these paths are sent to a
  remote Linux host over SSH and must always use forward slashes.
- test_ssh_bulk_upload.py: make test_staging_symlinks_mirror_remote_layout
  platform-agnostic — assert file existence and content instead of
  os.path.islink() + os.readlink(), since the staged entry may be a
  copy on Windows.

tests/tools/test_ssh_bulk_upload.py

@@ -90,7 +90,12 @@ class TestSSHBulkUpload:
         assert "/home/testuser/.hermes/credentials" in mkdir_str
 
     def test_staging_symlinks_mirror_remote_layout(self, mock_env, tmp_path):
-        """Symlinks in staging dir should mirror the .hermes-relative layout."""
+        """Staged file in staging dir should mirror the remote path structure.
+
+        On platforms where symlinks are available (Linux/macOS) the staged
+        entry is a symlink; on Windows it may be a regular copy.  Either way
+        the file must exist at the expected path and contain the right data.
+        """
         f1 = tmp_path / "local_a.txt"
         f1.write_text("content a")
 
@@ -105,11 +110,14 @@ class TestSSHBulkUpload:
                 # Capture the staging dir from -C argument
                 c_idx = cmd.index("-C")
                 staging_dir = cmd[c_idx + 1]
-                # Check the symlink exists
+                # Check the staged entry exists at the base-relative path
                 expected = os.path.join(staging_dir, "skills/my_skill.md")
                 staging_paths.append(expected)
-                assert os.path.islink(expected), f"Expected symlink at {expected}"
-                assert os.readlink(expected) == os.path.abspath(str(f1))
+                # File must exist (either as symlink or copy)
+                assert os.path.exists(expected), f"Expected staged file at {expected}"
+                # Content must match the source
+                with open(expected, "r") as fh:
+                    assert fh.read() == "content a"
 
             mock = MagicMock()
             mock.stdout = MagicMock()