gate(eligibility): minimum open-window before PRs against priority issues are gate-eligible

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Problem

Priority issues carry the highest payout, so assignment fairness matters most there. First-come pickup is only fair if everyone can see the issue before anyone can act on it: a PR opened moments after labeling means the window between "issue becomes valuable" and "issue is claimed" was effectively zero for everyone else.

Sizing

Contributor pickup on newly posted eligible issues is typically minutes, so the window only needs to exceed the visibility gap — long enough that anyone watching the repo has a fair chance to see the label, short enough not to slow normal throughput. Proposed default: 30 minutes, configurable per repo.

Scope

• Gate rule: a PR closing an issue labeled gittensor:priority is gate-eligible only if the label had been publicly present for the configured window (default 30m) before the PR was opened
• PRs arriving inside the window are not rejected outright: the gate holds them with a neutral comment stating the eligibility time, and they proceed normally once the window has elapsed — no penalty beyond waiting
• Window evaluated from label-event timestamps already available to the pipeline; enforcement decisions ledger-logged with rule id
• Docs: add the window to the same label-policy page as the author-eligibility rule, so contributors can see exactly when a priority issue opens for work
• Tests: PR opened inside the window → held, then eligible after elapse; PR opened after the window → unaffected; label re-application does not reset the clock for already-open PRs

Acceptance

Every priority issue has a knowable, uniform moment at which work on it becomes eligible; early PRs are held, not punished; enforcement is visible in the ledger.

Out of scope

Assignment/reservation mechanics (issues remain unassigned, first-eligible-merge wins as today).

[linear-code]

JSO-6801