fix(brainfs): refuse a file behind a symlinked directory for reads and writes (#173)

os.Root follows an intermediate directory symlink that stays inside the
brain. A dir symlink inside wiki/ pointing elsewhere in the brain let
brain_update and brain_tunnel rewrite an existing .md there (e.g. under
raw/), and session_log followed a symlinked sessions dir.

- WriteRegular, ReadRegular and ReadRegularMax require every directory
  above the file to be a real directory (realDir). Reads too: brain_tunnel
  read the redirected target, then rewrote the source before the target's
  write was refused; and brain_read/brain_get could read another part of
  the brain through such a path.
- session.Append refuses a sessions dir that is not a real directory.
- A directory swapped in after the check remains the #167 residual.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/brainfs/brainfs.go

@@ -128,6 +128,9 @@ func (b *Brain) ReadDirDirs(relDir string) ([]string, error) {
 // ReadRegular reads a brain-relative path, refusing anything that is not a
 // regular file, both before the open and on the opened handle.
 func (b *Brain) ReadRegular(rel string) ([]byte, error) {
+	if err := b.realDir(path.Dir(filepath.ToSlash(rel))); err != nil {
+		return nil, err
+	}
 	fi, err := b.root.Lstat(rel)
 	if err != nil {
 		return nil, err
@@ -154,6 +157,9 @@ func (b *Brain) readOpened(rel string) ([]byte, error) { return b.readOpenedMax(
 // ReadRegularMax is ReadRegular with a size limit, checked on the opened
 // handle before reading (max < 0 means no limit).
 func (b *Brain) ReadRegularMax(rel string, max int64) ([]byte, error) {
+	if err := b.realDir(path.Dir(filepath.ToSlash(rel))); err != nil {
+		return nil, err
+	}
 	fi, err := b.root.Lstat(rel)
 	if err != nil {
 		return nil, err
@@ -256,10 +262,18 @@ func (b *Brain) WalkRegular(relDir string, fn func(rel string) error, onErr func
 // O_NOFOLLOW (verified on go1.27), so the path is Lstat-ed through the root
 // first and the opened handle must be that same file (os.SameFile), which
 // also catches a swap between the two. O_NONBLOCK keeps a FIFO from blocking
-// the open. Intermediate in-brain symlinks are still followed (os.Root keeps
-// the write inside the brain; hyperguild#167).
+// the open. Every directory above the file must be a real directory
+// (hyperguild#173); a directory swapped in after that check is the #167
+// residual.
 func (b *Brain) WriteRegular(rel string, data []byte) error {
 	rel = filepath.ToSlash(rel)
+	// Every directory above the file must be real: os.Root follows an
+	// in-brain intermediate symlink, which would redirect the write
+	// elsewhere in the brain (hyperguild#173). A swap after this check is
+	// the #167 residual.
+	if err := b.realDir(path.Dir(rel)); err != nil {
+		return err
+	}
 	lfi, err := b.root.Lstat(rel)
 	if err != nil {
 		return err