ci: enable trusted publishing releases

[Puppo]

Adds a manually dispatched release workflow with patch, minor, and major choices, enabling approval-gated npm trusted publishing through the shared Fastify workflow.

The release job grants id-token: write and contents: write and pins fastify/workflows/.github/workflows/reusable-release.yml to b1a11bce713bc857e52b2231fec3c89071ad3135. The shared workflow uses the release environment, commits/pushes the version bump, publishes with npm provenance, and creates the GitHub Release. This JavaScript package needs no release build step.

Part of the JS-4 rollout batch, following the pilot in fastify/fastify-accepts#231.

Validation

Local validation with Node.js 24.21.0 and npm 11.19.0:

• npm install --ignore-scripts --no-audit --no-fund passed.
• npm run lint passed.
• npm test passed.
• Actionlint 1.7.12 passed; dispatch choices, permissions, reusable-workflow inputs, pinned SHA, and inherited release environment verified.
• npm pack: verified index.js, types/index.d.ts, and unchanged package version 11.0.0.

Admin setup before the first release

These settings are not changed by this PR and remain unverified:

• Configure the release environment with required reviewer team fastify/release and deployment access exclusively for branch main; check for extra branch/tag policies.
• Confirm branch protection/rulesets allow the shared workflow to push the version commit using GITHUB_TOKEN.
• Configure the npm GitHub Actions Trusted Publisher for @fastify/nextjs: label manual-gha-release, organization fastify, repository fastify-nextjs, workflow filename release.yml, environment release, Allow publish enabled.
• After review, setup, and merge, validate one real release: approval gate/reviewer notifications, package contents/provenance, version commit, tag, and GitHub Release. Use manual Discord notification if necessary.

No version bump, npm publication, release dispatch, or administrative settings/access changes were performed while preparing this draft.

Additional validation notes:

• The configured npm test builds the Next.js test/example application. This is independent of release packaging; no release:build hook was added.