fix(vectorstore): walk and read the brain through brainfs (#157, slice 3)

Sync walked wiki/ and knowledge/ with filepath.WalkDir and read each .md
with os.ReadFile. A symlinked note (Syncthing syncs symlinks) had its
target embedded into pgvector and served by semantic search; a FIFO
stalled the sync. The walk and read now go through brainfs: regular
files only, type checked on the opened handle.

- Chunks already embedded from a symlinked note are deleted on the next
  sync, because the note is no longer seen.
- A walk error still aborts Sync before the delete pass, so an unreadable
  subdir does not delete chunks of files that exist. A missing scan dir
  is skipped as before.
- brainfs gains Lstat, used for the mtime staleness check.

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

ingestion/internal/brainfs/brainfs_test.go

@@ -3,6 +3,7 @@ package brainfs
 import (
 	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"syscall"
@@ -139,3 +140,18 @@ func (c *countingReader) Read(p []byte) (int, error) {
 	c.n += n
 	return n, err
 }
+
+func TestLstatDoesNotFollowSymlinksAndRefusesEscapes(t *testing.T) {
+	dir := t.TempDir()
+	outside := filepath.Join(t.TempDir(), "o.md")
+	require.NoError(t, os.WriteFile(outside, []byte("o"), 0o644))
+	require.NoError(t, os.Symlink(outside, filepath.Join(dir, "l.md")))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+	fi, err := b.Lstat("l.md")
+	require.NoError(t, err)
+	assert.Equal(t, fs.ModeSymlink, fi.Mode().Type())
+	_, err = b.Lstat("../" + filepath.Base(filepath.Dir(outside)) + "/o.md")
+	assert.Error(t, err)
+}