ci: auto-assign PR author when none is set

[darkobas2]

What

Adds a small workflow that assigns the PR author as the assignee when a PR is opened without one, so no PR is left unassigned (helps board/filter/report visibility).

How

• Uses the official actions/github-script — no third-party/marketplace action, no external dependency.
• Triggers only on pull_request: opened.
• Skips if an assignee is already set (no duplicate, doesn't override an author-chosen assignee).
• Skips fork PRs (their token is read-only and the author isn't a collaborator, so assignment can't apply).
• Minimal permission: the built-in GITHUB_TOKEN with pull-requests: write, scoped to the run — no PAT, no GitHub App.

Note

This only populates the assignee field for tracking visibility — it is intentionally not a review-accountability mechanism. That would be CODEOWNERS, which is deferred until path → owner boundaries are defined.