chore(skills): add issue-sweep skill - #11583

[skip ci]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. (Repo tooling only; no issue.)
• You have signed the CLA.
• All existing tests are passing. (No source changes; CI skipped.)
• Tests for the changes are included. (Agent skill and scripts only.)

What is the current behavior?

There's no shared, repeatable way to triage the issue backlog. Old issues pile up, many for work that has already shipped but was never linked back.

What is the new behavior?

Adds the issue-sweep agent skill (.agent/skills/issue-sweep/) and lists it in AGENTS.md. It sweeps open issues oldest-first in batches:

1. scripts/fetch-batch.mjs fetches the next batch after a saved cursor (body, recent comments, labels, cross-referenced PRs) into a digest.
2. Each issue gets one bucket (shipped, obsolete, duplicate, abandoned, won't do in core, still valid, your call), backed by evidence from the code, git history or the thread.
3. The maintainer reviews a table of proposed actions and draft comments.
4. scripts/apply.mjs dry-runs by default. With --execute it closes or labels issues with their comments, skips anything that changed since the fetch, logs every action, and only advances the cursor when the whole batch applied.

State lives in ~/.cache/nativescript-issue-sweep/, so a rerun only picks up issues filed after the last sweep.

This was used on October 23, 2026 to review all 795 open issues: 566 closed (89 as already completed) and 229 kept and triaged.