go-live 7/9: triage and bulk-close the v1 backlog

[cliffhall]

Phase 7 of 9 in the v2 go-live runbook — see #1804 (§8). Depends on phase 6.

Note: this was originally planned as phase 6. It was swapped with the contribution-model change (#1820, now phase 6) so external pull requests are closed before this triage runs — otherwise new PRs keep arriving into a backlog being closed.

⚠️ The backlog is 125 open PRs, not ~30. gh pr list caps at 30 by default, which badly understates it; use --limit 500. All 125 were retargeted from main to v1/main before the tree swap (#1817), so their diffs and merge bases are intact. §8's warning about GitHub secondary rate limits definitely applies at this size.

⚠️ IRREVERSIBLE in practice — bulk closure is hard to walk back cleanly.

Close the open v1 backlog with an audit trail.

Tasks

• Triage before closing. Do not blanket-close by "not labeled v2" — some open v1 issues describe real bugs that still exist in v2, or security items we actually want. Split into port to v2 (relabel v2, add to board Add tab and approval flow for server -> client sampling #28) vs. close as deprecated.
• Apply a label (e.g. closed-v1-deprecated) as well as the comment, so the set stays queryable later. A comment alone is very hard to find again.
• Script the closure, but throttle — expect GitHub secondary rate limits on a few hundred closures.
• Do not lock the threads: leave comments open so someone can say "this still reproduces in v2".