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>

.mcp.json

@@ -0,0 +1,10 @@
+{
+  "mcpServers": {
+    "supervisor": {
+      "command": "/Users/mathias/dev/AI/supervisor/bin/supervisor-bridge",
+      "env": {
+        "SUPERVISOR_URL": "http://koala:30320/mcp"
+      }
+    }
+  }
+}