Add Dependabot auto-merge workflow

[Pigbibi]

Summary

• add a workflow_run gate that merges successful Dependabot dependency PRs after CI passes
• restrict auto-merge to Dependabot authors with the dependencies label
• allow both current GitHub author logins seen for Dependabot: dependabot[bot] and app/dependabot

Verification

• git diff --check
• PR Bump gnzsnz/ib-gateway from 10.37.1q to 10.46.1g #53 CI passed after the dependency test hardening

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Provide repo context before calling gh pr

gh pr list is executed without a checked-out repository and without --repo/GH_REPO, so on a fresh workflow_run runner this step can fail before any PR is resolved. GitHub CLI only lets you omit --repo when running inside a local Git repo with a GitHub remote; this workflow has no actions/checkout step, so the auto-merge path can fail for every run instead of merging Dependabot PRs.

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