feat: add assign-prs-to-author reusable workflow

[mattmillerai]

What

Adds a third reusable workflow to this repo: assign-prs-to-author.yml. It assigns every open PR with no assignees to its author, so unassigned PRs get routed back to whoever opened them. Bot-authored PRs (Dependabot, Renovate, app tokens) are skipped by default via the skip-bots input.

Extracted from a copy that currently lives in Comfy-Org/cloud, following the same thin-caller pattern as cursor-review and detect-unreviewed-merge.

Why a reusable workflow

It's generic CI housekeeping with zero repo-specific anything — pure github-script over context.repo. Hosting it here means consumer repos carry a small scheduled caller instead of a maintained copy (no drift).

Permissions

Uses the caller's GITHUB_TOKEN (no secrets to pass). The calling job must grant pull-requests: write and issues: write — assignees are set through the issues API.

Caller pattern

name: Housekeeping - Assign PRs to Author
on:
  schedule:
    - cron: "0 2 * * *"   # daily at 02:00 UTC
  workflow_dispatch:
jobs:
  assign:
    permissions:
      pull-requests: write
      issues: write
    uses: Comfy-Org/github-workflows/.github/workflows/assign-prs-to-author.yml@<sha>  # v1