fix(release): pass --tag next when publishing a pre-release

[Luis85]

Summary

The v0.8.0-rc.1 dispatch (run 25640431367) reached the npm publish step and failed with:

```
npm error You must specify a tag using --tag when publishing a prerelease version.
```

`npm publish` refuses to default a semver prerelease (`-rc.1`) to the `latest` dist-tag because that would silently demote stable consumers who pin `latest`. Without `--tag`, prereleases fail closed.

Parametrise the publish step on `inputs.prerelease`:

prerelease	publish args
`true`	`npm publish --provenance --tag next `
`false`	`npm publish --provenance ` (defaults to `latest`)

What the failed dispatch confirmed

Every gate before npm CLI ran green:

• Layer 1 readiness ✓
• Build claude-plugin in workflow ✓
• Layer 2 readiness ✓
• Confirm gate ✓
• Tarball provenance attestation ✓
• GitHub Release create (with asset upload) ✓
• npm publish CLI argument validation ✗ — this PR's fix

So the npmjs.com Trusted Publishing config and the OIDC token-mint path are reachable end-to-end. This fix unblocks the last gate before OIDC auth actually runs.

Test plan

• `npx vitest run tests/scripts/release-readiness.test.ts` — 36/36 pass.
• CI green on this PR.
• Post-merge: force-move `v0.8.0-rc.1` tag forward to new main HEAD, re-dispatch `release.yml --ref v0.8.0-rc.1 -f version=0.8.0-rc.1 -f prerelease=true -f publish_package=true -f dry_run=false -f confirm=0.8.0-rc.1`.
• Verify post-publish:
  ```
  npm view specorator@0.8.0-rc.1 --json | jq '{version: .version, attestations: .dist.attestations, dist_tags: ._npmUser}'
  npm view specorator dist-tags --json # should show "next": "0.8.0-rc.1", "latest" unchanged at "0.7.0"
  ```

Idempotency on re-dispatch

The previous failed dispatch created the `v0.8.0-rc.1` GitHub Release with the candidate tarball asset attached. The re-dispatch will hit the workflow's "promote in place" branch (step 9a):

• Detect existing Release → skip create.
• Asset already attached → skip upload (avoids clobber-then-fail data loss path).
• `gh release edit --draft=false --prerelease=true` → flag flip (no-op since current flags match).
• Proceed to npm publish step with `--tag next`.

🤖 Generated with Claude Code