chore(index): repin to the operator-held signing key

The bootstrap key generated during implementation was lost with its scratch
directory. Nothing shipped pinning it, so this is a straight replacement of
slot 0 rather than a rotation — the second slot stays reserved for a real one.

The private half now lives only in the INDEX_SIGNING_KEY secret, so CI is the
only thing that can sign. This push is what makes it do so: the committed
signature is still the old key's, and the verify self-check will fail loudly
if the secret does not match the public key pinned here.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/keys.ts

@@ -22,7 +22,7 @@ export const PUBKEY_PREFIX = "ed25519:";
  * `verify` defaults to this so a signed index can be checked with no arguments.
  */
 export const DEFAULT_PUBLIC_KEY =
-  "ed25519:n/KgBQSSDZqvyGHa8PkHWHZtV1zRXLk0BdQUi4/BD/w=";
+  "ed25519:qK18TE2qygIyObtMlHxUI/G1gKby8tPxuieAfnEKgYE=";
 
 /** Encode a node KeyObject public key as the host's `ed25519:<base64>` form. */
 export function encodePublicKey(key: KeyObject): string {