build: add nix devshell and pre-commit hooks

Problem: oil.nvim had no reproducible dev environment. The .envrc
set up a Python venv for the now-removed docgen pipeline, and there
were no pre-commit hooks for local formatting checks.

Solution: add flake.nix with stylua, selene, and prettier in the
devshell. Replace the stale Python .envrc with 'use flake'. Add
.pre-commit-config.yaml with stylua and prettier hooks matching
other plugins in the repo collection.

.envrc

@@ -1,3 +1 @@
-export VIRTUAL_ENV=venv
-layout python
-python -c 'import pyparsing' 2>/dev/null || pip install -r scripts/requirements.txt
+use flake