Adds `sign-daemon-release.sh release <secret.key> <version>`: zips each staged
prebuilt-binaries/dragonxd-{linux,mac,win}/ into dragonx-<version>-{linux-amd64,
macos,win64}.zip, signs each (detached ed25519), and prints the SHA-256 checksum
table for the release body. Platforms with no staged daemon are skipped; warns
if the signing key doesn't match the pubkey pinned in daemon_updater.h. Keeps the
existing keygen/pubkey/sign subcommands.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>