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>

.gitea/workflows/publish.yml

@@ -70,7 +70,7 @@ jobs:
       # auditor would use. Catches a key/host mismatch BEFORE it ships, rather
       # than as every host in the field silently rejecting the catalog.
       - name: Verify signature
-        run: bun run verify -- --pub "${{ vars.INDEX_PUBLIC_KEY || 'ed25519:n/KgBQSSDZqvyGHa8PkHWHZtV1zRXLk0BdQUi4/BD/w=' }}"
+        run: bun run verify -- --pub "${{ vars.INDEX_PUBLIC_KEY || 'ed25519:qK18TE2qygIyObtMlHxUI/G1gKby8tPxuieAfnEKgYE=' }}"
 
       - name: Commit signature to main
         run: |