fix(brain): context, brain_get and brain_read read through brainfs (#157, slice 2)

Option 1 of #157, slice 2. readExcerpt (brain_context), api.ReadNote
(brain_get / read-after-write) and notes.ReadScoped (brain_read) read
the Syncthing-shared brain by path. Under a symlink swap race each
leaked an outside file hundreds of thousands of times in 1.5s, including
brain_read, whose Lstat check was raced by the reopen-by-name; a FIFO
blocked os.ReadFile.

- All three read through brainfs (os.Root, regular files only,
  non-blocking open, type check on the handle).
- brainfs.ReadRegularMax checks a size limit on the opened handle;
  notes keeps its ErrTooLarge.
- New internal/brainfs/brainfstest: the shared swap-race harness
  (symlink and held-writer FIFO swappers, awaited before TempDir
  cleanup; AssertNoLeak / AssertNoHang) for this and later slices.

Tests red first: each reader under a symlink-swap race and a held-writer
FIFO race, plus static checks. Flipped: readExcerpt back to os.ReadFile
is red.

Refs #157

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

ingestion/internal/notes/read_root_test.go

@@ -0,0 +1,31 @@
+package notes
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs/brainfstest"
+)
+
+// hyperguild#157 slice 2: ReadScoped (brain_read) Lstat-checked the final
+// component, then reopened by name with os.ReadFile: a swap after the check
+// was followed, and a FIFO blocked the read. It now reads through brainfs.
+func TestReadScopedIsConfinedToRegularFilesInTheBrain(t *testing.T) {
+	brain := t.TempDir()
+	facts := filepath.Join(brain, "wiki", "w", "facts")
+	require.NoError(t, os.MkdirAll(facts, 0o755))
+	p := filepath.Join(facts, "r.md")
+	read := func() string {
+		b, _ := ReadScoped(brain, "wiki/w/facts/r.md", ScopeNotes)
+		return string(b)
+	}
+	require.NoError(t, os.WriteFile(p, []byte("inside"), 0o644))
+	assert.Equal(t, "inside", read())
+	brainfstest.AssertNoLeak(t, p, "inside", 1500*time.Millisecond, read)
+	brainfstest.AssertNoHang(t, p, "inside", 1500*time.Millisecond, 6*time.Second, func() { _ = read() })
+}