docs: refresh AGENTS.md for current Rust and runtime surfaces

[aaf2tbz]

Summary

Refresh AGENTS.md to match the current post-v0.59.1 repository and Rust backend.

Changes

• Document the expanded Rust backend/MTSP modules and route groups.
• Describe the current vkd3d-proton M12 default and the recognized-but-not-yet-distinct DXMT rollback configuration accurately.
• Correct runtime paths, split bundle names, release source, verification, and repair behavior.
• Add current diagnostics, D3D12 SDK gates, CI workflows, hook/tool guidance, version-bump files, and existing-surface safety rules.
• Clarify stateful launch/doctor checks, bundle side effects, and common runtime pitfalls.

PR Readiness

This is documentation-only; no runtime behavior, compatibility, bottle, or version surfaces changed.

• Compatibility verified with at least one real game (not applicable: documentation-only)
• No hardcoded paths, secrets, or absolute /Users/... paths introduced
• Config/rules TOML validated if configs/mtsp-rules.toml or DLL maps changed (not applicable)
• Version triple (CMakeLists.txt, Cargo.toml, package.json, package-lock.json) in sync if version bumped (not applicable)
• Bottle/runtime migration and launch behavior preserved (documentation-only)
• Docs / compatibility matrix updated for user-facing changes
• Regression test added for each bug fix (not applicable: no bug fix)

Local checks

• git diff --check passed.
• Relative AGENTS.md links were checked and all resolve.
• python3 tools/ci/check-doc-freshness.py ran; it reports the existing unrelated missing Updated: header in docs/OPENGL-2-4-PLAN.md.
• Independent documentation review completed with no actionable findings.
• Rust, native, Electron, and game-runtime tests were intentionally skipped because only AGENTS.md changed.

Risk

Documentation-only. No executable or packaged assets changed.

[aaf2tbz]

Reopening to rerun the readiness workflow after adding the documentation-only checklist exception.