#1408 docs: a short "Contributing a change or report" section in AGENTS.md and a test-requests list

Description

## Title
Issue: Resolves #909

## Summary
**Docs only, no engine code: an eight-line contributing section in AGENTS.md and docs/TEST_REQUESTS.md, a five-row table of hardware wanted that points at docs/COMMUNITY_BENCHMARKS.md.** As offered in #909 and accepted there; the wording is yours to edit or cut.

## What changed
- AGENTS.md: new section "Contributing a change or report" (search first, use the issue forms, one change per PR, opt-in with the default path unchanged, build every backend a file touches, change a default only where measured faster, reports from other hardware follow the benchmark format). The rules come from your review comments on #390, #492, #583, #598 and #663.
- docs/TEST_REQUESTS.md (new): unequal NVIDIA pair, 3-4 NVIDIA GPUs, Pascal (sm_61), Linux host with 32 GB RAM or less, long soaks. Each row is taken from what you wrote on earlier threads, plus Pascal, which I can run.

## Extra Notes
I drafted this with an AI assistant (Claude Code) and checked every issue number against the repo myself.