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_test.go

@@ -326,3 +326,28 @@ func TestReadDirDirsListsOnlyRealSubdirsAndRefusesSymlinkedComponents(t *testing
 	_, err = b.ReadDirDirs("alias")
 	assert.ErrorIs(t, err, ErrNotRegular)
 }
+
+// hyperguild#173: a file reached through a symlinked directory inside the
+// brain is refused for reads and writes, so a caller-supplied path cannot be
+// redirected to another part of the brain.
+func TestReadAndWriteRegularRefuseAFileBehindASymlinkedDirectory(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "raw"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "raw", "v.md"), []byte("untouched"), 0o644))
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki"), 0o755))
+	require.NoError(t, os.Symlink(filepath.Join("..", "raw"), filepath.Join(dir, "wiki", "alias")))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+
+	_, err = b.ReadRegular("wiki/alias/v.md")
+	assert.ErrorIs(t, err, ErrNotRegular)
+	_, err = b.ReadRegularMax("wiki/alias/v.md", 100)
+	assert.ErrorIs(t, err, ErrNotRegular)
+	assert.ErrorIs(t, b.WriteRegular("wiki/alias/v.md", []byte("x")), ErrNotRegular)
+	got, _ := os.ReadFile(filepath.Join(dir, "raw", "v.md"))
+	assert.Equal(t, "untouched", string(got))
+	got, err = b.ReadRegular("raw/v.md")
+	require.NoError(t, err, "positive control: the real path still reads")
+	assert.Equal(t, "untouched", string(got))
+}