Temporarily pull brain upload checklist (keep guide) until memory fix ships

[rezakhadjavi]

Temporarily remove the brain upload checklist

Why: Customers have uploaded files through the "Your checklist" tab on the public how-to-build-the-brain page and we have lost that data twice while the temporary-memory handling is still being fixed. Pulling the intake until Ioana's memory-pipeline fix lands removes the risk. Temporary and fully reversible.

Approved by: Kyra (admin) in #kyra-runneth-triage, for Josh (CSM). Ioana is co-owner on the memory-fix side.

Changes

1. use-case-library-site/server/src/how-to-build-the-brain.html
   • Removed the "Your checklist" tab and the entire #panel-checklist upload form.
   • Removed the now-dead tab-switching / upload JS (kept the /* __CSM_INJECT__ */ marker so the server-side inject stays a safe no-op).
   • The guide is untouched, including "The 10 most impactful uploads, in order." The page still teaches people how to build the brain; it just no longer collects files.
2. use-case-library-site/server/src/index.ts
   • POST /api/brain-checklist now returns a friendly 503 "temporarily paused" message by default, so a cached page or a #checklist deep link cannot still submit and lose data.
   • Gated behind BRAIN_CHECKLIST_PAUSED (paused unless set to 0), so re-enabling is a one env-var flip or removing the block.

Reverting

Revert this PR (or set BRAIN_CHECKLIST_PAUSED=0) once the memory fix has shipped.

Notes for review

• A couple of sentences in the guide still reference "section 6 of the checklist." Left in place to honor the ask to keep the guide unchanged. Easy to soften in a follow-up if you'd prefer.
• Merging to main triggers the Railway deploy, so leaving the merge to you.