docs: reproduce-before-you-fix instructions for contributors and AI agents (#4254)

* docs: add reproduce-before-you-fix instructions for contributors and AI agents

Adds .github/REPRODUCING.md and references it from the bug report
template, the PR template, and AGENTS.md: reproduce the issue before
fixing (provoke it with loops/threads/timing stress and temporary
instrumentation when needed), package the reproducer as a toggleable
component, and verify the fix against it with before/after evidence.

* docs: reproduce-before-fix applies to everyone, reference it from CONTRIBUTING.md