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]

Move app-token secrets out of this Dependabot event

This job only runs for dependabot[bot] PRs under pull_request_target, and GitHub's secret restrictions say that in this case workflow secrets are not available, aside from the read-only GITHUB_TOKEN. Because the new app-token step gets both required inputs from secrets.*, every patch/minor Dependabot PR will pass empty app credentials into actions/create-github-app-token and fail before the approve/merge steps; the app-token creation needs to run from a non-Dependabot-triggered handoff such as workflow_run/dispatch, or use credentials available in this event.

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