test: add lefthook as git hook

lefthook.yaml

@@ -0,0 +1,14 @@
+---
+skip_output: [meta, success]
+no_tty: true
+
+commit-msg:
+  commands:
+    lint:commitmsg:
+      run: cat {1} | convco check --from-stdin
+pre-push:
+  parallel: false
+  commands:
+    check:nix:
+      tags: check
+      run: nix flake check