feat!: probes link to the panel; signed release updates for container nodes [CI]

Probes connect to the panel directly and get the agent list as desired state.
CI signs the image digest of agent and probe releases, the panel forwards the
manifest, and a host updater verifies it with a local public key, pulls by
digest and rolls back when the new version is unhealthy.

packaging/update/release-key.pub.pem

@@ -0,0 +1,4 @@
+-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEfw7EkNcl0FZqOETK+Vt55RMZ9K0q
+YOrtO3Vso/QNdSMmepQlx1Q3v+KkN7ro3AY3I6DT2Q2txJu5pS7qzutSQA==
+-----END PUBLIC KEY-----