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]

Avoid secrets in this Dependabot pull_request_target job

This job only runs for Dependabot-authored pull_request_target events, and GitHub documents that for that case the GITHUB_TOKEN is read-only and secrets are not available: https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-on-actions#restrictions-when-dependabot-triggers-events. As a result, these newly required app credentials resolve to empty values on the intended patch/minor Dependabot PRs, so actions/create-github-app-token cannot produce steps.app-token.outputs.token and the approve/auto-merge steps never run.

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