Add Nix dev environment, replace husky with git hooks

Sets up a Nix-based dev environment (fenix + crane) and swaps the
husky pre-commit hook for a tracked .githooks directory wired via
core.hooksPath.

Husky and lint-staged weren't doing much. The only tracked JS is
index.js and index.d.ts, both auto-generated by NAPI-RS with
eslint-disable and prettier-ignore at the top. lint-staged was
formatting code that opts out of linting, while the Rust source
got nothing. The new hook just runs `just check`, which runs
`nix flake check` (fmt, clippy, build).

Cargo.lock is now tracked. Crane vendors git deps from it so it
has to be visible to Nix, and for a library shipped via npm
tracking the lockfile is the right call anyway as binaries are
produced.

Author: amackillop

.envrc

@@ -0,0 +1 @@
+use flake

.githooks/pre-commit

@@ -0,0 +1,2 @@
+#!/usr/bin/env bash
+just check

.github/workflows/CI.yml

@@ -26,7 +26,7 @@ concurrency:
   cancel-in-progress: true
 jobs:
   lint:
-    name: Lint
+    name: CI
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -45,14 +45,13 @@ jobs:
       - name: Install dependencies
         run: yarn install
 
-      - name: Oxlint
-        run: yarn lint
-
-      - name: Cargo fmt
-        run: cargo fmt -- --check
+      - name: Install just
+        uses: taiki-e/install-action@v2
+        with:
+          tool: just
 
-      - name: Clippy
-        run: cargo clippy
+      - name: Check
+        run: just ci
   build:
     strategy:
       fail-fast: false

.gitignore

@@ -117,7 +117,6 @@ dist
 #Added by cargo
 
 /target
-Cargo.lock
 
 *.node
 .pnp.*