swarmctl: agent watch follows an agent's terminal rows

Reads GET /api/agents/{name}/term/stream over the controller socket or the
gateway and prints one line per row. The gateway path uses a client without
the total-request timeout so the stream isn't cut after 30 seconds.

Closes #6160

swarmctl/src/main.rs

@@ -34,6 +34,7 @@ mod permissions;
 mod remote;
 mod schedule;
 mod users;
+mod watch;
 
 use std::fs::{self, File, Permissions};
 use std::io::{self, Write as _};
@@ -292,6 +293,15 @@ enum AgentVerb {
     /// No approval gate guards this: the caller is root on the controller's
     /// host, or logged in through the swarm domain's SSO gate.
     SetState(AgentSetStateArgs),
+    /// Follow an agent's terminal rows live: the newest retained rows, then
+    /// each new one, one line per row, until Ctrl-C or the stream ends.
+    ///
+    /// The swarm-level counterpart of `hivectl agent watch`, read from the
+    /// swarm-controller rather than from the agent's own web UI, so it works
+    /// from any machine with `--swarm` and doesn't need the agent running to
+    /// show what it last did. Fails when the swarm has never heard from the
+    /// agent.
+    Watch(AgentWatchArgs),
     /// Show the model, effort, subagent models and subagent cap the swarm sets
     /// for an agent, or change the model, effort and subagent models.
     ///
@@ -445,6 +455,14 @@ struct AgentSetStateArgs {
     socket: ControllerSocketArg,
 }
 
+#[derive(Args)]
+struct AgentWatchArgs {
+    /// Agent name.
+    agent: String,
+    #[command(flatten)]
+    socket: ControllerSocketArg,
+}
+
 #[derive(Args)]
 struct AgentCreateArgs {
     /// Name for the new agent: 1–63 characters of `[a-z0-9-]`.
@@ -726,6 +744,12 @@ fn main() -> Result<()> {
             let target = args.socket.resolve(&remote)?;
             agent::set_state(&target, &args.agent, &args.hive, args.state, args.yes)
         }
+        Verb::Agent {
+            command: AgentVerb::Watch(args),
+        } => {
+            let target = args.socket.resolve(&remote)?;
+            watch::watch(&target, &args.agent)
+        }
         Verb::Agent {
             command: AgentVerb::Inference { command },
         } => inference(&remote, command),