ci(automerge): delegate to the fleet reusable automerge workflow

[cmarko89]

Summary

Replaces this repo's hand-maintained automerge.yml with a thin caller to the fleet reusable workflow Emkraan/.github/.github/workflows/reusable-automerge.yml@main (Emkraan/.github#17). Only the triggers stay here. All merge logic now lives in one place and cannot drift.

Why

13 repos had drifted into 5 automerge variants with 3 readiness rules. On repos gating on CLEAN only, a draft marked ready after its checks went green stranded open: the running automerge job pins the PR UNSTABLE, native auto-merge is refused, and no later event fires (Emkraan/apollo#1037). The shared rule is: CLEAN, or UNSTABLE where every non-green check belongs to automerge itself. Non-required red checks still block. Verified live on Emkraan/.github#17, which was readied from draft and self-merged with UNSTABLE only from automerge itself.

Changes in the caller

• pull_request gains closed, so linked issues are closed explicitly on merge (GitHub skips this for bot merges)
• Permissions gain issues: write plus read-only checks, statuses and actions, for reading the check rollup
• workflow_run lists every workflow in this repo that runs on pull_request
• Runner unchanged: forge for private repos, GitHub-hosted for public repos (fork-PR safety, per the 2026-09-19 runner design)

Standard: Apollo docs/standards/github-repo-standard.md Section 8.