Add the gh-stack dev skill

Vendor the gh-stack agent skill verbatim from github/gh-stack at
2bd699a544a09cb5c45a013d03416e0894b0454e with its MIT license, so coding
agents working on Chopin can create and manage stacked pull requests.
dprint skips the directory so the files stay identical to upstream and
can be refreshed by copying.

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
User-preference: Group related infrastructure PRs into their own GitHub stack, separate from feature stacks
Co-authored-by: Alex Lavaee <lavaman131@github.com>