ci(github): use app token for dependabot merges

[ncipollina]

Summary

Updates the Dependabot auto-merge workflow to use the LayeredCraft GitHub App token for approving and enabling auto-merge. This avoids using GITHUB_TOKEN for the merge operation so downstream workflows can run after Dependabot auto-merged changes land on main.

Changes

• Adds a SHA-pinned actions/create-github-app-token step for semver patch/minor Dependabot PRs.
• Uses the generated app token as GH_TOKEN for gh pr review --approve and gh pr merge --auto --squash.
• Keeps dependabot/fetch-metadata on the default GITHUB_TOKEN.

Validation

• Reviewed workflow syntax and token usage.
• No build required; workflow-only change.

Notes for Reviewers

Requires the Actions secrets DEPENDABOT_AUTOMERGE_APP_ID and DEPENDABOT_AUTOMERGE_PRIVATE_KEY to be available to this repository.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use a trigger that can read the app credentials

In this pull_request_target job, the only runs that reach this step are Dependabot-authored PRs (github.event.pull_request.user.login == 'dependabot[bot]'). GitHub documents that Dependabot-initiated pull_request_target runs with that condition get a read-only GITHUB_TOKEN and no secrets, so these new app-id/private-key inputs evaluate empty and actions/create-github-app-token fails before the approve/auto-merge steps can use the app token. Move the token generation to a Dependabot context that exposes the credentials, such as a pull_request workflow using Dependabot secrets, or another trusted two-step workflow.

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