ci: sign plugins-index.json in-repo, drop worker signing dependency

Pair with the worker-side simplification on the librefang PR — the
worker is now a pure transport (no key material, no signing) and this
repo's CI takes over signature production.

scripts/sign-plugins-index.mjs reads REGISTRY_PRIVATE_KEY from a GitHub
Actions secret, signs plugins-index.json with Ed25519, and writes
plugins-index.json.sig alongside it. Aborts loudly when the secret is
missing so a misconfigured CI can't silently ship an unsigned payload.

The workflow now runs build → sign → commit (.json + .sig) → push →
poke worker /refresh. The worker fetches the committed .json + .sig
verbatim and stores both — the daemon then verifies against the
embedded pubkey it ships with.

Closes PR review CRITICAL #1: the worker is no longer a sign-anything
oracle reachable via REGISTRY_REFRESH_TOKEN. Trust root is now this
repo's branch protection + Actions secret scope, not a token any CI
job that can talk to stats.librefang.ai can use to mint signatures.

Note: the keypair was rotated as part of this change (PR not yet
merged so no daemon TOFU pins exist). New pubkey:
  ClGa0Ucap8NdrKAy1rw9Tt6A9I8eg4zJ53+xIuKMuq0=
The plugins-index.json.sig committed here is signed with the matching
new private key, in lockstep with the daemon EMBEDDED_REGISTRY_PUBKEY
constant and all three worker [vars] entries.

plugins-index.json.sig

@@ -0,0 +1 @@
+efERqfz7oGh0AoBrnVKocOUjOUR5h3a8vkIcvqd+HP+k3Rr99diJsExKwXAogfz4TpTWPn0rqLjVgQpP8MxXBw==