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/pipeline/refs.go

@@ -2,7 +2,6 @@
 package pipeline
 
 import (
-	"os"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -53,8 +52,7 @@ func injectSourceRefs(pages []wiki.Page, inventory map[wiki.PageType][]wiki.Entr
 		if !ok {
 			continue
 		}
-		diskPath := filepath.Join(brainDir, "wiki", string(pt), slug+".md")
-		b, err := os.ReadFile(diskPath)
+		b, err := readBrainFile(brainDir, "wiki/"+string(pt)+"/"+slug+".md")
 		if err != nil {
 			continue
 		}