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>

README.md

@@ -230,7 +230,7 @@ PR cannot get anything signed.
 The public key currently pinned in the host:
 
 ```
-ed25519:n/KgBQSSDZqvyGHa8PkHWHZtV1zRXLk0BdQUi4/BD/w=
+ed25519:qK18TE2qygIyObtMlHxUI/G1gKby8tPxuieAfnEKgYE=
 ```
 
 `bun run verify` defaults to this key, so anyone can audit a published index