docs: prepare production submission launch copy

[kim-em]

Summary

• make the stable LeanEval submission page the primary submission path
• retain issue intake as the explicit fallback during the 28-day launch overlap
• remove disabled/prelaunch wording from the README and submission-related issue templates

Launch gate

Draft only: do not merge until the production lifecycle canary has passed and public server intake is ready to be announced. Merging this PR changes public-facing submission guidance.

Validation

• exact head 00b91e4f0943f0212d9184a951d3a08a1ab4244e passed required CI run 33323032862
• branch is rebased directly on protected main 7e977ab32fa56be5b7f29933b3b8b02f9823d4b1; its diff is limited to launch copy in the README and issue templates
• git diff --check
• parsed all issue-template YAML and checked the basic GitHub issue-template shape
• verified the stable submission page and issue-intake fallback URLs are reachable
• checked the edited files for stale disabled/prelaunch wording