ci: add contributor tier labels and external PR triage workflows

[tcharchian]

Summary

Adds two GitHub Actions workflows:

• contributor-labels: assigns a single contributor:* label to a PR author based on their number of merged PRs:
  • contributor:first-time — 0 merged PRs (their first PR)
  • contributor:new — 1–10 merged PRs
  • contributor:regular — 11–30 merged PRs
  • contributor:established — more than 30 merged PRs
  • Runs on PR opened/reopened and on merge; keeps exactly one tier label; auto-creates the labels if missing.
• triage-external-prs: when a PR is opened by a non-collaborator (not OWNER/MEMBER/COLLABORATOR), adds it to project Add a basic participant: #7 and sets its Status to new.

Setup notes

• triage-external-prs requires a PROJECTS_TOKEN repository secret (PAT with project + repo, or fine-grained with Projects read/write + Pull requests read). Already added to repo secrets.
• Project number, owner, status field name and value are configurable via env at the top of the workflow (defaults: owner = repo owner, project Add a basic participant: #7, field Status, value new).

Test plan

• Open a PR from a non-collaborator account and confirm it lands in project Add a basic participant: #7 with Status = new.
• Confirm a first-time contributor's PR gets contributor:first-time.
• Confirm tier label updates after merges.

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