refactor: replace Git LFS with runtime model download from Hugging Face

Remove ~170MB of model files from the repository (LFS + committed).
Models are now downloaded at runtime from Hugging Face on first use
and cached to the OS cache directory with progress reporting and retries.

- Add internal/models/download.go: runtime downloader with retry, progress bar, checksums
- Remove go:embed for ONNX models (keep tokenizers embedded)
- Use file-based ONNX session loading instead of byte-slice
- Add scripts/download-models.sh for dev/CI model setup
- Update Makefile with setup-models target
- Update workflow-prepare.sh to download models in CI
- Set lfs: false in all CI workflows
- SHA256: bge=828e14..., cross-encoder=5d3e70...

Makefile

@@ -23,6 +23,10 @@ all: build
 setup-libs:
 	@./scripts/download-onnx-libs.sh all
 
+# Download ONNX models from Hugging Face (for local dev, skips if present)
+setup-models:
+	@./scripts/download-models.sh
+
 # Update version in committed plugin metadata
 update-version:
 	@if [ -f .claude-plugin/plugin.json ]; then \
@@ -182,12 +186,14 @@ uninstall: stop-worker
 	@echo "Uninstallation complete!"
 
 # Run tests (with FTS5 support)
-test: setup-libs
-	go test $(BUILD_TAGS) -v -race ./...
+# CLAUDE_MNEMONIC_MODEL_DIR points to a common model directory for tests.
+# Run setup-models first to download models from Hugging Face.
+test: setup-libs setup-models
+	CLAUDE_MNEMONIC_MODEL_DIR=$$(pwd)/testdata/models go test $(BUILD_TAGS) -v -race ./...
 
 # Run tests with coverage (with FTS5 support)
-test-coverage: setup-libs
-	go test $(BUILD_TAGS) -v -race -coverprofile=coverage.out ./...
+test-coverage: setup-libs setup-models
+	CLAUDE_MNEMONIC_MODEL_DIR=$$(pwd)/testdata/models go test $(BUILD_TAGS) -v -race -coverprofile=coverage.out ./...
 	go tool cover -html=coverage.out -o coverage.html
 	@go tool cover -func=coverage.out | tail -1