feat(scripts): fold model downloads into update.sh, skip if already present

Today's embedding-server crash-loop (missing nomic-embed-text GGUF) was a
manual step nobody ran. update.sh now runs both downloader profiles
itself, every time, before bringing services up -- no separate command to
remember.

- docker-compose.yml: downloader/downloader-embedding commands gain a
  `test -f ... && skip || curl ...` guard, so re-running update.sh never
  re-downloads an existing model file.
- scripts/update.sh: runs both profiles after image pull/build, before
  service recreation.
- scripts/download-model.sh removed -- folded in, redundant standalone
  script.
- README.md / docs/memory-knowledgebase.md updated accordingly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018WHfjWrSEcGhCoeu6dQfDa

scripts/update.sh

@@ -63,6 +63,10 @@ docker compose pull --ignore-buildable
 echo "==> rebuilding local-build services"
 docker compose build --pull
 
+echo "==> ensuring models are downloaded (skips already-present files)"
+docker compose --profile tools run --rm downloader
+docker compose --profile tools run --rm downloader-embedding
+
 echo "==> bringing up litellm (needed to mint virtual keys below)"
 docker compose up -d --wait litellm-db litellm