chore: migrate biome.json to 2.x schema

Root pins @biomejs/biome@^2.4.15 but biome.json still used the 1.8.3
schema, so biome 2.x refused to parse it (`files.ignore` was renamed to
`files.includes` with negated globs). This broke `code:fix` and the
per-package `lint` scripts entirely.

Migrated via `biome migrate` and adopted biome's recommended folder-ignore
form for nix/store. Faithful config migration only — no rule or behavior
changes, and biome is not run in CI so this has no pipeline impact.

Author: coderdan

biome.json

@@ -1,7 +1,13 @@
 {
-  "$schema": "https://biomejs.dev/schemas/1.8.3/schema.json",
+  "$schema": "https://biomejs.dev/schemas/2.4.15/schema.json",
   "files": {
-    "ignore": ["**/package.json", ".turbo/", "dist", "**/nix/store/**"]
+    "includes": [
+      "**",
+      "!**/package.json",
+      "!**/.turbo/",
+      "!**/dist",
+      "!**/nix/store"
+    ]
   },
   "formatter": {
     "indentStyle": "space",