CI: welcome first-time contributors with intro-issue nudge

[brendancol]

Reason or Problem

The repo has a New contributor introduction issue template at .github/ISSUE_TEMPLATE/new-contributor.md. It asks newcomers about their AI tooling, geospatial background, and what they'd like to work on, so maintainers can point them at fitting issues. Right now nothing surfaces it to first-time PR authors - they can open a PR without knowing the template exists.

Proposal

Add .github/workflows/welcome-contributor.yml. On pull_request_target: [opened]:

1. Skip drafts, same-repo PRs, and PRs from members/owners/collaborators.
2. Check whether the PR author already has an issue titled Contributor intro: ... (any state).
3. If not, post one friendly PR comment linking to the prefilled template URL.

Modeled on .github/workflows/copilot-review.yml - same event type and same gating approach, since the same-repo author_association quirk noted in that file's inline comment applies here too.

Drawbacks

A new contributor who filed an intro issue under a different title would still get the nudge. Rare and harmless.

Alternatives

• Mention the intro issue in CONTRIBUTING.md. Cheap but easy to miss.
• A GitHub App / bot. Overkill for one comment.