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

## 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.

Trace the gh stack submit and gh stack link entry points, then inspect the existing gh-stack.remote and gh-stack.rerere-declined configuration handling to find where the hardcoded footer is appended. Decide which requested opt-out fits the CLI conventions; done means the footer remains enabled by default and is omitted when the opt-out is configured or passed.

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