Reset Repo Assist memory branch

[dsyme]

Summary

• store Repo Assist state on the fresh memory/repo-assist-memory branch
• remove the legacy in-workflow memory migration step
• restore the one-file persistence limit and document that old memory branches are left untouched

Validation

• gh aw compile workflows/repo-assist.md --strict --validate --no-emit
• git diff --check

Security review

The compiler reported the existing repo-memory validation script as an addition to the tracked safe-update manifest. The validator itself is unchanged from main; this change adds no secret, action, permission, redirect, or network destination. The custom validator remains the persistence gate for the single notes.json file.