Realign with template: carry review-loop contract, Copilot runbook, and markdownlint config

[ptr727]

Drift found while auditing derived repos against ptr727/ProjectTemplate, per the template's verbatim-carry contract (introduced in ptr727/ProjectTemplate#124):

• Missing .github/copilot-instructions.md review runbook - the file does not carry the "GitHub Copilot Review Runbook" (the GraphQL requestReviews mechanics, the copilot-pull-request-reviewer login note). An agent here has no in-repo pointer to the reliable Copilot mechanics.
• Missing AGENTS.md "PR Review Etiquette" section - the provider-agnostic review-loop contract is not present.
• Missing .markdownlint-cli2.jsonc - no shared lint config, so the davidanson IDE extension and CLI markdownlint-cli2 can disagree. Copy the template's verbatim.

.editorconfig/.gitattributes and the App-token client-id/rulesets are already aligned - good.

Scope check: this repo is intentionally NuGet-only and simplified. The .markdownlint-cli2.jsonc carry is unambiguous; please confirm whether the full review-loop contract + runbook are wanted here before restructuring AGENTS.md, or close as intended divergence.