refactor(studio): cheaper blob Exists and accurate filesystem docs

Exists checked a blob by opening and closing it. After the symlink
check, one Lstat is enough.

NewFS said it needs exclusive creation, but publication renames over an
identical blob, which is harmless because the name is the hash. LinkTo
on the filesystem store copies and never links; say so.

Assisted-by: Claude Code:claude-sonnet-5-5
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

core/services/studio/workspace/blob/fs.go

@@ -20,7 +20,9 @@ type fsStore struct {
 }
 
 // NewFS stores immutable blobs in a service-controlled directory. Shared
-// filesystems must support coherent exclusive creation, rename and directory sync.
+// filesystems must support coherent atomic rename and directory sync. Two
+// writers of the same hash may both rename; the bytes are identical, so the
+// last rename wins harmlessly.
 func NewFS(dir string) (Store, error) {
 	abs, err := filepath.Abs(dir)
 	if err != nil {
@@ -207,19 +209,24 @@ func (s *fsStore) Open(_ context.Context, sha string) (io.ReadSeekCloser, error)
 	}
 	return f, err
 }
-func (s *fsStore) Exists(ctx context.Context, sha string) (bool, error) {
-	f, err := s.Open(ctx, sha)
-	if errors.Is(err, workspace.ErrNotFound) {
+func (s *fsStore) Exists(_ context.Context, sha string) (bool, error) {
+	if !validSHA(sha) {
 		return false, nil
 	}
-	if err != nil {
+	p := shaPath(s.root, sha)
+	if err := s.safe(p); err != nil {
 		return false, err
 	}
-	return true, f.Close()
+	_, err := os.Lstat(p)
+	if errors.Is(err, os.ErrNotExist) {
+		return false, nil
+	}
+	return err == nil, err
 }
 
-// LinkTo deliberately copies. The same UID can chmod a read-only hard link and
-// mutate its inode, so a hard link is not an isolation boundary for tool files.
+// LinkTo copies the blob and never links it, despite the name it has in Store.
+// The same UID can chmod a read-only hard link and mutate its inode, so a hard
+// link is not an isolation boundary for tool files.
 func (s *fsStore) LinkTo(ctx context.Context, sha string, dir *os.Root, name string) error {
 	if !filepath.IsLocal(name) {
 		return workspace.ErrInvalid