fix(cli): display system push messages without decryption

System messages (watch_triggered, mcp_deployed, peer_joined, etc.)
have senderPubkey='system' with empty ciphertext. The push handler
now formats them as readable plaintext instead of failing to decrypt.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claudemesh-cli",
-  "version": "0.8.7",
+  "version": "0.8.8",
   "description": "Claude Code MCP client for claudemesh — peer mesh messaging between Claude sessions.",
   "keywords": [
     "claude-code",