failure-summary: an installation token can reach a check-run summary via JSON.stringify(cause)

[debuggingfuture]

A step failure whose cause is neither a string nor an Error is rendered into the check-run summary with JSON.stringify:

https://github.com/fractalboxdev/flare-dispatch/blob/e8a1300/apps/dispatcher/src/failure-summary.ts#L59-L67

const cause =
  typeof e.cause === "string"
    ? e.cause
    : e.cause instanceof Error
      ? e.cause.message
      : JSON.stringify(e.cause);

CheckoutFailed is exactly that shape — it carries cause: Schema.Unknown (packages/core/src/errors.ts:12-16), so the whole object is serialized, nested cause included.

Why that is a credential path

gitClone authenticates the clone URL with a GitHub App installation token before handing it to the sandbox:

https://github.com/fractalboxdev/flare-dispatch/blob/e8a1300/packages/runtime-cf/src/sandbox-clone-url.ts#L21-L23

https://x-access-token:<token>@github.com/<owner>/<name>.git

If box.gitCheckout(cloneUrl, …) rejects with anything that carries the URL — and git routinely echoes the remote it failed to reach — that URL becomes CheckoutFailed.cause, and stepFailedMd stringifies it into a check-run summary, which is visible to anyone who can see the check. The token is short-lived (~1h) but live at the moment it is published.

Observed

PR #89 produced this summary for real, from a genuine checkout failure:

**Step `checkout` failed**:

CheckoutFailed: { "repo": "fractalboxdev/flare-dispatch", "sha": "36cd4922d9a1…", "cause": Error: Network connection lost. }

Here the inner cause was a benign network error, so nothing leaked. It demonstrates the path, not the leak: the same rendering with a git-originated cause is what publishes the token.

Suggested fix

Two independent changes, either of which closes it; both are worth having:

1. Do not serialize an unknown cause. stepFailedMd should fall back to a fixed string rather than JSON.stringify — an unrecognised cause is precisely the case where nothing is known about what its fields hold. runs/org-spec-audit.ts took this approach for its own describe() in org-spec-audit: the estate sweep for questions specs can't answer — and two fixes it needed to run #89.
2. Match CheckoutFailed explicitly and render repo + sha only. Those are the whole diagnosis; the git error tail adds nothing worth carrying a credential to get.

Worth also scrubbing x-access-token:<token>@ at the summary boundary as a backstop, since the URL can reach it through causes other than CheckoutFailed.

Notes

Found while reviewing #89; unrelated to that PR's diff, which is why it is filed separately rather than fixed there.