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/api/update_redirect_test.go

@@ -0,0 +1,29 @@
+package api_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/api"
+)
+
+// hyperguild#173: a directory symlink inside wiki/ that points elsewhere in
+// the brain redirected brain_update's write (os.Root follows in-root
+// intermediate symlinks). Every directory above the note must be real.
+func TestUpdateNoteRefusesANoteBehindASymlinkedDirectory(t *testing.T) {
+	brain := t.TempDir()
+	victim := filepath.Join(brain, "raw", "facts", "v.md")
+	require.NoError(t, os.MkdirAll(filepath.Dir(victim), 0o755))
+	require.NoError(t, os.WriteFile(victim, []byte("---\ntitle: v\n---\nuntouched\n"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(brain, "wiki", "w"), 0o755))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "raw", "facts"), filepath.Join(brain, "wiki", "w", "alias")))
+
+	_, _, _, err := api.UpdateNote(brain, api.UpdateNoteOptions{Path: "wiki/w/alias/v.md", Content: "replaced"})
+	assert.Error(t, err)
+	got, _ := os.ReadFile(victim)
+	assert.Contains(t, string(got), "untouched")
+}