fix(ingestion): embed sync also scans brain/knowledge/ + logs per-item errors

The embed sync goroutine only walked brain/wiki/. brain/knowledge/ (112
curated entries, per CLAUDE.md the most-important brain content) had zero
coverage in brain_embeddings — vector retrieval was blind to it. Hybrid
BM25 + pgvector retrieval would never surface a curated knowledge entry
via the vector arm.

Extract the per-root walk into a loop over a small subdir list and add
"knowledge" alongside "wiki". scanDirs is package-level so it stays a
single source of truth for what gets embedded.

Also log each failing item's path + error string from StartSync.
Previously only the aggregate count was logged, so a persistent
`errors=1` per cycle was opaque. With per-item warnings, the actual
ollama "input length exceeds the context length" surface immediately.

Refs gitea/mathias/infra#37 (this commit covers the knowledge/ scan
bug; the long-file chunking bug is a separate change.)

ingestion/internal/vectorstore/sync_test.go

@@ -117,6 +117,20 @@ func TestSync_SkipsIndexFiles(t *testing.T) {
 	assert.NotContains(t, store.upserts, "wiki/a/_index.md")
 }
 
+func TestSync_ScansKnowledgeDir(t *testing.T) {
+	dir := t.TempDir()
+	writeNote(t, dir, "wiki/a/facts/x.md", "x")
+	writeNote(t, dir, "knowledge/2026-05-19-koala-gpu-setup.md", "knowledge body")
+
+	store := &stubStore{known: map[string]struct{}{}}
+	emb := stubEmbedder{vec: make([]float32, 768)}
+	res, err := vectorstore.Sync(context.Background(), dir, store, emb)
+	require.NoError(t, err)
+	assert.Equal(t, 2, res.Added)
+	assert.Contains(t, store.upserts, "wiki/a/facts/x.md")
+	assert.Contains(t, store.upserts, "knowledge/2026-05-19-koala-gpu-setup.md")
+}
+
 func TestSync_NoOpWhenComponentsNil(t *testing.T) {
 	dir := t.TempDir()
 	writeNote(t, dir, "wiki/a/facts/x.md", "x")