CONTRIBUTING says what an assignment is for: the first approve-worthy PR wins

Context

@tobiu ruled on 2026-09-24, relayed by @neo-opus-grace on #18985, that assignment is behavioural guidance, not a gate: "contributors can just create PRs without assignments … in this case we are not the ones to blame when collisions happen. we still accept the first PR for a given ticket (if approve-worthy)."

CONTRIBUTING.md §3 item 3 says something else. "please add a comment there and get the ticket assigned to you. Otherwise multiple contributors could work on the same item in parallel" reads as a requirement. Today an agent pipeline quoted it and withdrew from #19140. Meanwhile a real first-timer's claim on #19143 sat unanswered for 27 h.

The Problem

• The text does not say what an assignment is for, namely protection from a collision. It also does not say that the first approve-worthy PR wins.
• A contributor who skips the claim cannot tell that the risk is theirs. A contributor who waits cannot tell why waiting is worth it.

The Fix

Rewrite §3 item 3 of CONTRIBUTING.md to state the rule as it is:

• comment to claim, and we assign the first person who asks;
• you can open a PR unassigned, but we merge the first approve-worthy PR for an issue, so an assignment is what protects your time.

The 7-day reassignment sentence stays. The text makes no response-time promise, because claims do not reach a maintainer automatically yet (the #18985 notification leaf).

Acceptance Criteria

• AC-1 §3 item 3 states the claim rule (first claimant assigned, first approve-worthy PR merged, assignment as protection) and makes no response-time promise.
• AC-2 Item 1's good-first-issue sentence stays consistent with item 3.

Out of Scope

• The claim notification (the #18985 leaf that @neo-opus-grace takes).
• The curated-issue template sentence, banked for the 10-01 batch.

Related

#18985 (parent) · #19140 · #19143

Sweeps (at filing):

• Live latest-open: latest open issues; none equivalent. A keyword search found nothing.
• A2A: @neo-opus-grace's #18985 messages at 19:03, 19:07 and 19:09Z propose this wording. She offered the notification or check leaf, not this one.
• Own assignments: none equivalent.

Origin Session ID: 101d2ce9-9f43-4f5a-9ae2-3c75bf8f6fcf