feat: Added ping command

claude.md

@@ -395,7 +395,18 @@ service Vantage {
 
 `CommandStream` is the only streaming RPC: the agent authenticates once with `AgentReady`, then the server pushes `ServerCommand`s and the agent replies with `CommandResult`, `StepResult`, or `StepOutputChunk`.
 
-`ServerCommand` variants: `GenerateKeyCmd`, `DeleteKeyCmd`, `UpdateAgentCmd`, `ApplyUpdatesCmd`, `RunStepCmd`, `CleanupWorkspaceCmd`.
+`ServerCommand` variants: `GenerateKeyCmd`, `DeleteKeyCmd`, `UpdateAgentCmd`, `ApplyUpdatesCmd`, `RunStepCmd`, `CleanupWorkspaceCmd`, `OpenProxyCmd`, `PingCmd`.
+
+**`PingCmd` is a liveness beat, and it is not redundant with gRPC keepalive.**
+The server sends one every 20s on an otherwise idle command stream; the agent
+treats 70s of silence as a dead stream and reconnects. Keepalive cannot do this
+job behind an L7 proxy: the agent's HTTP/2 connection terminates at the proxy,
+which answers pings on its own behalf, so a control-plane pod that dies leaves
+the agent blocked in `Recv` on a stream that never delivers another message and
+never errors — commands dispatched into it are silently lost while `SyncKeys`
+keeps succeeding and the fleet list still shows the server `active`. The agent's
+watchdog arms only **after** it has seen a first ping, so an older server that
+sends none is treated as working rather than put into a reconnect loop.
 
 Key-state polling stays on the 30s `SyncKeys` interval. Full message definitions live in `proto/vantage/v1/vantage.proto`.