Part of GitHub stack #190 (infrastructure).

Adds the `gh-stack` agent skill to `.agents/skills/gh-stack/`, so coding agents working on Chopin can create and manage GitHub stacked pull requests with the [`gh stack`](https://github.com/github/gh-stack) CLI extension.

This is the second PR in an infrastructure stack, on top of #185 (Bun 1.4.2). The pluggable-harness work for #181 is a separate stack.

- The skill is vendored verbatim from `github/gh-stack` at `2bd699a544a09cb5c45a013d03416e0894b0454e`: `SKILL.md` and `references/{commands,stack-design,troubleshooting}.md`, plus the upstream MIT `LICENSE`. - `dprint.json` excludes `.agents/skills/gh-stack/**`, so the files stay identical to upstream and a refresh is a plain copy. dprint would otherwise restyle two markdown emphasis markers. - Like `testing-pr-previews`, this is a development skill living directly in `.agents/skills/`. It is not a product skill under `skills/`.

One note for this repository: if your `origin` remote uses an SSH host alias (for example `workgit:githubnext/chopin`), `gh stack` can't infer the repository. Set `GH_REPO=githubnext/chopin` on the command.

Assistant-model: Claude Opus 5.5 Assistant-workflow: inline Assistant-verification: bun run ci passed: dprint, oxlint, tokens with the vendored skill excluded from formatting Assistant-verification: diff -r against upstream passed: skill files identical to github/gh-stack@2bd699a apart from the added LICENSE Assistant-verification: relative link check passed: every .md link in the skill resolves Assistant-verification: gh stack link passed: GH_REPO=githubnext/chopin gh stack link created stack #188 from #185-#187 with the workgit SSH alias remote User-preference: Group related infrastructure PRs into their own GitHub stack, separate from feature stacks Co-authored-by: @lavaman131