Add a way to turn off the "Stack created with GitHub Stacks CLI" footer

gh stack submit と gh stack link のエントリーポイントを追跡し、既存の gh-stack.remote と gh-stack.rerere-declined の設定処理を調べて、ハードコードされたフッターがどこで追加されているかを確認します。要求されているオプトアウトのうち、CLI の規約に適合するものを決定します。完了条件は、フッターがデフォルトで有効なままになり、オプトアウトが設定されているか渡された場合に省略されることです。

Problem

Every PR that gh stack submit or gh stack link creates gets this line appended to the bottom of the description:

Stack created with GitHub Stacks CLI • Give Feedback 💬

There's currently no flag, git config, or environment variable that turns it off. I checked the binary (strings on the compiled extension) — the only gh-stack.* git-config keys it reads are gh-stack.remote and gh-stack.rerere-declined. The footer HTML is a hardcoded string, not templated or gated by anything.

Why this matters more than a cosmetic PR-description line

On repos that use GitHub's squash-merge with "default to pull request title and description" — a common, often-default setting (squash_merge_commit_message: "PR_BODY" in the repo API) — the PR body becomes the literal squash commit message when the PR merges.

That means this footer doesn't just live in the PR UI temporarily. It gets written into the permanent, immutable git log of the target branch, on every single stacked PR, forever.

Prior art

Claude Code ships a documented includeCoAuthoredBy setting in settings.json specifically to let users disable the "🤖 Generated with Claude Code" / "Co-Authored-By: Claude" trailer it otherwise adds to commits and PRs — for the same underlying reason: that text ends up permanently in shared git history, not just in a UI a user can dismiss. The same reasoning applies here.

Request

A way to suppress the footer, e.g.:

• A gh-stack.footer=false git config value (matching the existing pattern of gh-stack.remote), or
• A --no-footer flag on submit/link

Either would work — the point is just having some opt-out.