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.go

@@ -53,6 +53,12 @@ func (b *Brain) Rel(path string) (string, error) {
 	return filepath.ToSlash(rel), nil
 }
 
+// Lstat stats a brain-relative path through the root without following a
+// final symlink.
+func (b *Brain) Lstat(rel string) (fs.FileInfo, error) {
+	return b.root.Lstat(filepath.ToSlash(rel))
+}
+
 // ReadRegular reads a brain-relative path, refusing anything that is not a
 // regular file, both before the open and on the opened handle.
 func (b *Brain) ReadRegular(rel string) ([]byte, error) {