feat: pause an agent's turn loop without stopping its container

A paused agent keeps its container, its claude session and its
dashboard/todo servers up, but stops driving turns. Messages queue
unacked and are drained on resume.

The whole protocol is a single marker file, `<harness>/paused`. That
directory is already a bind-mount shared between host and container, so
both sides just stat the same path: the harness reads it to decide
whether to drive a turn, hive-c0re reads it to render the badge and
writes/removes it for `hivectl pause|resume`. No new wire protocol, no
container round-trip, and it is sticky across restarts by construction.

Not calling `recv_next` while paused *is* the queueing semantic, so
there is no fencing to get wrong: reminders buffer in their unbounded
channel, the todo `Notify` permit coalesces, and a `request_next_turn`
that raced the pause survives because the gate sits above
`self_continue.take()`.

Graceful stop is handled host-side rather than in the harness: a paused
agent provably has no turn in flight, so `run_signal` skips the fence
entirely instead of eating the full `GRACEFUL_STOP_TIMEOUT` waiting for
a checkpoint turn that will never run.

`paused` is reported on `ContainerView` / `AgentStatusRow` for the
dashboard, orthogonal to `running` and reported for stopped containers
too.

Closes: hyperhive/hyperhive issue 2271

hive-c0re/src/job_queue/exec.rs

@@ -497,7 +497,18 @@ async fn run_stop_for_update(
 
 /// Set the graceful fence + kick so the harness sees it promptly and
 /// runs its one stop-checkpoint turn.
+///
+/// Skipped entirely for a paused agent: its loop parks on the pause
+/// marker without polling the broker, so it would never observe the
+/// fence and the downstream drain would just burn
+/// `GRACEFUL_STOP_TIMEOUT`. Safe because the harness tests the marker at
+/// the top of its loop — a paused agent has no turn in flight, so there
+/// is nothing to checkpoint.
 fn run_signal(coord: &Arc<Coordinator>, claim: &Claim, ctx: &Ctx<'_>) -> NodeOutput {
+    if hive_types::Ident::parse(&claim.agent).is_ok_and(|a| Coordinator::is_paused(&a)) {
+        ctx.step("graceful stop: agent paused, nothing to drain");
+        return NodeOutput::default();
+    }
     ctx.step("graceful stop: signalling agent");
     coord.mark_graceful_stop(&claim.agent);
     coord.kick_agent(&claim.agent, "graceful stop requested");