feat: fix NIP-07 signing to use node Nostr key, add test script

Added node.nostr-sign RPC that uses the node-level Nostr key (matching
getPublicKey), fixing pubkey mismatch where identity.nostr-sign used a
different key. Updated appLauncher to call node.nostr-sign. Added
nostr_sign_hash() to nostr_discovery.rs. Created test-nip07.sh with
11 automated checks (INSTALL-02).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/api/rpc/mod.rs

@@ -300,6 +300,7 @@ impl RpcHandler {
             "node.tor-address" => self.handle_node_tor_address().await,
             "node.nostr-publish" => self.handle_node_nostr_publish().await,
             "node.nostr-pubkey" => self.handle_node_nostr_pubkey().await,
+            "node.nostr-sign" => self.handle_node_nostr_sign(params).await,
             "node-nostr-verify-revoked" => self.handle_node_nostr_verify_revoked().await,
 
             // Encrypted peer handshake (NIP-44)