Update readme to point to proving repo.

[leo-starkware]

Type

• feature
• bugfix
• dev (no functional changes, no API changes)
• fmt (formatting, renaming)
• build
• docs
• testing

Description

Breaking changes?

• yes
• no

---

This change is 

[cursor]

PR Summary

Low Risk
Documentation-only README update with no code, security, or behavioral changes.

Overview
Adds a short repository migration notice at the top of README.md, stating that development moves to starkware-libs/proving from end of July 2026.

^{Reviewed by Cursor Bugbot for commit af580d6. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.86%. Comparing base (9960934) to head (af580d6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #412   +/-   ##
=======================================
  Coverage   67.86%   67.86%           
=======================================
  Files          42       42           
  Lines        5459     5459           
=======================================
  Hits         3705     3705           
  Misses       1754     1754           

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[yuvalsw]

@yuvalsw reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on leo-starkware).