build: use latest pnpm (12.x) managed by mise

- mise.toml: pnpm = latest
- pnpm 12 approves build scripts via allowBuilds in pnpm-workspace.yaml
- drop packageManager pin (mise is the version source; pnpm would
  self-downgrade to it) and the dead yarn pin in the root package.json

mise.toml

@@ -1,4 +1,4 @@
 [tools]
 python = "3.14"
 uv = "latest"
-pnpm = "10.33.0"
+pnpm = "latest"