chore: use legacy packageManager field to work around changesets issue

[mizdra]

Summary

After #435 migrated to devEngines.packageManager, the Release workflow started failing (log). changeset publish runs npm publish internally (changesets/changesets#965), and npm rejects the command because it does not match devEngines.packageManager: pnpm.

This PR reverts to the legacy packageManager field as a workaround. npm does not validate this field, so changeset publish works again. pnpm still auto-manages its own version via this field, so the pnpm@11 migration is kept.

Once changesets/changesets#2097 is released, we can migrate back to devEngines.

How to verify

• Before this change, running any npm command (e.g. npm view pnpm version) inside the repo failed with EBADDEVENGINES — the same check that broke changeset publish. After this change, it succeeds.
• pnpm install --frozen-lockfile succeeds and pnpm 11.9.0 is selected.

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: f5a81fa

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