docs: retire issue-intake fallback copy after cutoff

[kim-em]

Summary

• make the stable authenticated submission page the only documented intake path
• remove the expired Issue Form overlap, fallback, and retirement notice from the benchmark README and security guide
• make the repository Issue picker, stale-link redirect template, and problem-report guidance server-only
• add a focused regression test rejecting retired Issue URLs, wording, and timestamps in all three Issue configuration files

Merge blockers

NO-GO to merge now. This copy accompanies the post-cutoff retirement in leanprover/lean-eval-submissions#1654 and leanprover/lean-eval-leaderboard#95. Merge it only after:

• the announced cutoff at 2026-09-30T06:57:10Z has passed;
• the overlap, notice, incident, adoption, stability, final-delta, and readiness gates pass;
• every pre-cutoff Issue run has drained and the final corpus is reconciled;
• Issue intake has been retired through the ordered submissions change; and
• this branch is rebased onto current protected main and all required checks pass.

The mutable execution runbook must be updated with observed retirement facts at execution time, not prospectively in this draft.

Validation

• complete Python unit suite: 34 passed
• focused Issue-template regression: passed
• git diff --check: clean
• stale-copy scan over README, SECURITY, and Issue configuration/templates: clean
• hosted CI: https://github.com/leanprover/lean-eval/actions/runs/33835402318 passed