ci: use GitHub App token for prerelease workflow

[Hweinstock]

Description

Switch the prerelease-tarball workflow from secrets.GITHUB_TOKEN to a GitHub App token, consistent with all other workflows in the repo.

The default workflow token was insufficient because we need access to the private repo, see: https://github.com/aws/agentcore-cli/actions/runs/26293331790

Related Issue

N/A

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe): CI fix

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Workflow-only change — validated YAML syntax locally.

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the
terms of your choice.

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[github-actions]

Package Tarball

aws-agentcore-0.14.2.tgz

How to install

gh release download pr-1369-tarball --repo aws/agentcore-cli --pattern "*.tgz" --dir /tmp/pr-tarball
npm install -g /tmp/pr-tarball/aws-agentcore-0.14.2.tgz

[agentcore-cli-automation]

Looks good to merge. The change mirrors the GitHub App token pattern already used in release.yml, release-main-and-preview.yml, sync-from-public.yml, sync-preview.yml, and strands-command.yml (same actions/create-github-app-token@v1, same vars.APP_ID / secrets.APP_PRIVATE_KEY), so it's consistent with the rest of the repo. The rationale (default GITHUB_TOKEN lacking access to the private agentcore-l3-cdk-constructs repo during npm run bundle) checks out.

Minor (non-blocking) observation: the top-level permissions: contents: write block is now effectively dead since the release create/delete uses the App token instead of GITHUB_TOKEN. Safe to leave as-is or drop in a follow-up — not worth blocking on.