fix: make sync-vsix pick newest .vsix and clean stale ones

sync-vsix now uses ls -t to pick the newest .vsix by mtime instead of
alphabetically first, avoiding breakage when multiple .vsix files exist.
make clean now removes old .vsix files to prevent stale versions from
being embedded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

Makefile

@@ -69,13 +69,13 @@ sync-lint-rules:
 	done; \
 	if [ $$changed -gt 0 ]; then echo "Synced $$changed lint rule file(s)"; fi
 
-# Sync VS Code extension (.vsix) for embedding
+# Sync VS Code extension (.vsix) for embedding — picks newest .vsix by mtime
 sync-vsix:
-	@if [ -f vscode-mdl/vscode-mdl-*.vsix ]; then \
-		src=$$(ls vscode-mdl/vscode-mdl-*.vsix | head -1); \
+	@src=$$(ls -t vscode-mdl/vscode-mdl-*.vsix 2>/dev/null | head -1); \
+	if [ -n "$$src" ]; then \
 		if [ ! -f cmd/mxcli/vscode-mdl.vsix ] || ! cmp -s "$$src" cmd/mxcli/vscode-mdl.vsix; then \
 			cp "$$src" cmd/mxcli/vscode-mdl.vsix; \
-			echo "Synced vscode-mdl.vsix"; \
+			echo "Synced vscode-mdl.vsix ($$src)"; \
 		fi; \
 	elif [ ! -f cmd/mxcli/vscode-mdl.vsix ]; then \
 		echo "Warning: No .vsix found. Creating empty placeholder."; \
@@ -181,6 +181,7 @@ grammar:
 # Clean build artifacts
 clean:
 	rm -rf $(BUILD_DIR)
+	rm -f vscode-mdl/vscode-mdl-*.vsix
 	go clean
 
 # Build VS Code extension (.vsix) with build-time version info