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

hivectl/src/cli.rs

@@ -500,6 +500,22 @@ pub enum AgentsCmd {
         #[arg(long)]
         no_wait: bool,
     },
+    /// Park an agent's turn loop, leaving the container running.
+    ///
+    /// The harness stops driving turns but keeps serving its web UI and
+    /// MCP daemons, so the container, its mounts and its warm caches stay
+    /// up while it burns no tokens. Inbox messages queue unacked and the
+    /// backlog drains on `resume`. Sticky: it survives a restart, and
+    /// pausing a stopped agent makes it come up paused.
+    Pause {
+        /// Agent name.
+        name: String,
+    },
+    /// Resume a paused agent — it drains whatever queued up while parked.
+    Resume {
+        /// Agent name.
+        name: String,
+    },
     /// Spawn a new agent container directly, bypassing the approval queue.
     ///
     /// Operator-on-the-host only; use `request-spawn` for an approval-gated