ci: gate npm publish with an environment

[JoviDeCroock]

Summary

• split the release workflow into a release job and an environment-gated publish job
• move id-token: write onto the gated publish path instead of the release PR path
• add an unpublished-package check so stable publishes only run when the current package versions are not already on npm
• keep the prerelease/canary path separate where the repo already had one

Why

• matches the OIDC + environment-gated publishing pattern used in preactjs/signals and preactjs/preset-vite
• reduces the permissions carried by the release PR job
• makes the stable npm publish path explicit and reviewable

Verification

• parsed the workflow YAML locally
• ran the unpublished-package helper locally against the repo

[changeset-bot]

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Latest commit: eefe6f4

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