chore: delegate dependabot-auto-merge.yml to org reusable template

[kayodebristol]

Replaces the byte-identical local copy of dependabot-auto-merge.yml with a thin
caller into plures/.github's reusable template (workflow_call).

No behavior change: same triggers, same permissions, policy logic now lives in
one place instead of ~59 duplicated copies.

Depends on plures/.github#21 (adds the workflow_call trigger to the shared
template) - do not merge this before openclaw#21 lands.

Part of github-actions:ci-lockin-reduction epic.

[Copilot]

Pull request overview

Delegates this repo’s Dependabot auto-merge workflow to the org-level reusable workflow in plures/.github, aiming to eliminate duplicated policy logic across many repositories.

Changes:

• Replaces the in-repo Dependabot auto-merge job implementation with a reusable workflow call.
• Keeps the same pull_request_target trigger types and token permissions in the caller workflow.
• Adds a short header comment directing future policy edits to the shared org template.

[Copilot]

Restored the full local workflow body from the v4 policy commit. The org template's zerox.outputs.breaking != 'true' guard contradicts openclaw's "merge whenever possible" directive, so delegation to it was the wrong approach here. The local workflow now again auto-merges 0.x-breaking non-security bumps (CI-gated via --auto), with only non-security direct:production semver-major bumps held for human review.

Fix: fix: restore local dependabot-auto-merge workflow; org template blocks 0.x-breaking auto-merge unlike v4 policy