[codex] Fix contributor fork and pull request workflow

[BigDella]

Summary

• correct the README clone URL so contributors start from the canonical repository
• document the fork -> origin and canonical -> upstream workflow for SSH and HTTPS
• add exact branch push, PR creation, and common error recovery commands
• make the PR template verify the canonical base and contributor-fork head

Root cause

The README pointed at Obiajulu-gif/vaultquest, which redirects to Obiajulu-gif/vaultquest-archive. That repository is outside the Vaultquest/vaultquest fork network. Contributors following the README could not push there and GitHub could not use those repositories as heads for pull requests to the canonical project.

Validation

• git diff --check
• repository permission/settings audit: public, forking enabled, no repository interaction limit, no branch ruleset blocking PRs
• external fork PRs are currently accepted by GitHub
• node ./scripts/validate-docs.js was run; it reports only pre-existing broken links in Prisma migration docs and docs/USER_GUIDE.md, none in the changed files