fix: enable auto-assign (fix path + drop broken triage label call)

[baxyz]

Summary

• Second file with the same misplacement bug as the manual-fallback workflows (fix: adopt trigger-website-update, fix manual-fallback workflow paths #7): workflows/auto-assign.yml (repo-root, not .github/workflows/) was never registered as a runnable workflow — this repo's own issues/PRs have never been auto-assigned.
• Rather than just moving the existing bespoke implementation, replaced it with the same reusable-auto-assign.yml wrapper pattern already used by action/typescript/website/devcontainer's auto-assign.yml — for two reasons:
  • The old implementation's "Add labels to issue" step calls addLabels with ['triage'], but no triage label exists in this repo (checked via gh api repos/helpers4/.github/labels) — moving it as-is would have made every issue-opened run fail immediately.
  • Reusing the proven reusable workflow (already exercised by 4 other repos) is more consistent than keeping a diverging bespoke implementation that predates the standardized pattern.

Test plan

• After merge, open a test issue/PR to confirm auto-assign actually fires (never worked before)

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