fix(ingestion): always append .md extension to written filenames

brain_write with a custom filename omitted the .md extension, causing
search to skip the file (search.go filters on HasSuffix .md).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ingestion/internal/api/handler.go

@@ -99,7 +99,11 @@ func (h *Handler) Write(w http.ResponseWriter, r *http.Request) {
 		finalContent = fm.String() + req.Content
 	}
 
-	dest := filepath.Join(rawDir, filepath.Base(filename))
+	base := filepath.Base(filename)
+	if !strings.HasSuffix(base, ".md") {
+		base += ".md"
+	}
+	dest := filepath.Join(rawDir, base)
 	if err := os.WriteFile(dest, []byte(finalContent), 0o644); err != nil {
 		h.logger.Error("write failed", "err", err)
 		http.Error(w, "write error", http.StatusInternalServerError)