cleanup: retire legacy issue submission intake after cutoff

[kim-em]

Summary

• retire the legacy GitHub Issue Form and issue-triggered submission lane
• preserve the complete server archive, evaluation, lifecycle-State, recording, and result path
• remove issue-only reconciliation, validation, failure-comment, and documentation machinery
• retain the cutoff guard/classifier and final-delta machinery inert until their later cleanup

Stack and merge blockers

This draft is stacked on the exact head of #1650 (1ac11c24e818c67995002b4cee2a15f5adfec276) but targets upstream main so branch protection and hosted CI exercise the full eventual delta.

The matching user-facing retirement changes are prepared as
leanprover/lean-eval#624 and
leanprover/lean-eval-leaderboard#95.

NO-GO to merge now. Before merge, all of the following must be true:

• the announced 2026-09-30T06:57:10Z cutoff and required overlap/notice/readiness gates have passed;
• pre-cutoff issue runs are drained and the final inventory/delta is reconciled and promoted;
• feat: freeze legacy issue intake at the final cutoff #1650 and the bounded final-delta chain have been merged in their required order;
• this branch is rebased onto then-current protected main and independently re-reviewed;
• the two matching user-facing retirement PRs are rebased, green, and ready for the same retirement operation;
• current-state docs are updated at retirement time so they no longer describe issue intake as live;
• all required checks pass on the final exact head.

Validation at this draft head

• full Python suite: 1,012 passed, 2 skipped
• focused server/archive/cutoff suite: 34 passed
• Ruff, actionlint, action-pin audit, and diff checks: clean
• independent review: GO to open as draft; explicitly NO-GO to merge before the blockers above