ci: add triage automation workflow

[bdero]

Summary

Adds .github/workflows/triage.yaml to power the new weekly triage process.

PR-only flavor — only PRs are added to the queue (issues here aren't part of the meeting). On pull_request_target.opened it applies the needs-triage label so new PRs land in the org-level Triage project.

pull_request_target is used (not pull_request) so labeling works on PRs from forks. The workflow does not check out PR code, so it's safe.

Context

One of five PRs rolling out the new triage workflow across shorebird, _shorebird, dart-sdk, flutter, and docs. The label needs-triage was just created in this repo.

Test plan

• Merge → open a test PR → confirm needs-triage is auto-applied

[bdero]

Closing — we decided to drop PR labeling and rely on age-based saved views instead. See https://github.com/orgs/shorebirdtech/projects/25.