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/classification.go

@@ -16,9 +16,10 @@
 package classification
 
 import (
+	"errors"
 	"fmt"
-	"os"
-	"path/filepath"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
+	"io/fs"
 	"strings"
 
 	"gopkg.in/yaml.v3"
@@ -110,9 +111,20 @@ type rawConfig struct {
 func Load(brainDir string) (*Config, error) {
 	cfg := &Config{wings: map[string]Level{}, repos: map[string]Level{}}
 
-	data, err := os.ReadFile(filepath.Join(brainDir, "classification.yaml"))
+	// Read through the brain root, regular files only (hyperguild#157): a
+	// symlinked or special file is not trusted as the config, which is a hard
+	// error like a malformed one.
+	bfs, err := brainfs.Open(brainDir)
+	if errors.Is(err, fs.ErrNotExist) {
+		return cfg, nil
+	}
 	if err != nil {
-		if os.IsNotExist(err) {
+		return nil, fmt.Errorf("open brain: %w", err)
+	}
+	defer func() { _ = bfs.Close() }()
+	data, err := bfs.ReadRegular("classification.yaml")
+	if err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
 			return cfg, nil
 		}
 		return nil, fmt.Errorf("read classification.yaml: %w", err)