fix(pipeline,classification): read the brain through brainfs (#157, slice 6)

The ingest pipeline read wiki/sources/<slug>.md and schema.md (both go
into the LLM prompt), the pages it revises, and the pages it adds
source refs to, with os.ReadFile; BackfillRefs listed wiki/sources with
os.ReadDir. A symlinked file's content reached the prompt and the
written pages (1.6M leaks in the race), and a FIFO hung the ingest.
classification.Load (the I1 sovereignty levels) read
classification.yaml the same way and hung startup on a FIFO.

- pipeline: readSourcePage, loadSchema, reviseOrCreate, injectSourceRefs
  read through readBrainFile (brainfs ReadRegular); reviseOrCreate takes
  the brain dir and reads page.Path, a missing page is still created and
  a non-regular one is an error. BackfillRefs lists with ReadDirRegular
  and reads with ReadRegular.
- classification.Load: read through brainfs; a symlinked or special
  classification.yaml is a hard error (as a malformed one is); an absent
  file or brain dir still yields the default config.

Tests use in-brain targets and the [[slug|title]] link form the pipeline
actually follows (bare [[slug]] links are ignored by it, which made a
first draft of the backfill test vacuous).

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

ingestion/internal/classification/root_test.go

@@ -0,0 +1,46 @@
+package classification
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// hyperguild#157 slice 6: Load read classification.yaml with os.ReadFile. It
+// sets the I1 sovereignty levels; a symlinked file must not be trusted as the
+// config (a hard error, as for a malformed file), and a FIFO must not hang
+// startup.
+func TestLoadRefusesASymlinkedOrFIFOClassificationFile(t *testing.T) {
+	ok := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(ok, "classification.yaml"), []byte("wings:\n  w: confidential\n"), 0o644))
+	_, err := Load(ok)
+	require.NoError(t, err, "positive control")
+
+	linked := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(linked, "real.yaml"), []byte("wings:\n  w: public\n"), 0o644))
+	require.NoError(t, os.Symlink("real.yaml", filepath.Join(linked, "classification.yaml")))
+	_, err = Load(linked)
+	assert.Error(t, err, "an in-brain symlink is not trusted as the config")
+
+	fifo := t.TempDir()
+	require.NoError(t, syscall.Mkfifo(filepath.Join(fifo, "classification.yaml"), 0o644))
+	w, err := os.OpenFile(filepath.Join(fifo, "classification.yaml"), os.O_RDWR, 0)
+	require.NoError(t, err)
+	defer func() { _ = w.Close() }()
+	done := make(chan error, 1)
+	go func() { _, err := Load(fifo); done <- err }()
+	select {
+	case err := <-done:
+		assert.Error(t, err)
+	case <-time.After(3 * time.Second):
+		t.Fatal("Load hung on a FIFO")
+	}
+
+	_, err = Load(t.TempDir())
+	assert.NoError(t, err, "an absent file is still not an error")
+}