feat(release): enable npm staged publishing (#215)

* feat(release): enable npm staged publishing

Switch npm publish to npm stage publish so releases go through an
approval checkpoint before becoming publicly available on the registry.
Bumps the npm pin to 11.15.0, the minimum version that supports the
stage command.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(ci): bump npm pin in build workflow to 11.15.0

Matches the engines constraint added in the staged publishing change;
without this the build workflow fails with EBADENGINE on npm ci.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Tobias Wennergren <tobias@fal.ai>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>