docs: delete two false cost claims about the boot sweep's drains

Both said a whole-hive graceful stop costs ONE `GRACEFUL_STOP_TIMEOUT`
in total because drains overlap. That is only true for a power op. In a
rebuild subtree the brace holds the build slot across the whole subtree,
drain included, so the boot sweep's per-agent drains serialise and the
sweep costs one timeout per wave of `buildSlots`.

Deleted rather than corrected. The right cost statement depends on an
operator knob and belongs in docs/coordinator.md if it belongs anywhere;
a comment that has to hedge about a config value is the kind that goes
stale silently. A comment saying nothing beats one that lies.

hive-c0re/src/job_queue/exec.rs

@@ -19,9 +19,7 @@ use crate::power::{ReconcileAction, reconcile_action};
 /// Max time `Drain` waits for the harness to run its stop-checkpoint
 /// turn before falling back to the hard stop. Generous — a checkpoint
 /// turn can take a while — but bounded so a wedged agent never blocks
-/// the stop indefinitely. Drains hold no build slot, so a whole-hive
+/// the stop indefinitely.
-/// graceful stop overlaps every agent's drain instead of serialising
-/// N × this timeout.
 pub const GRACEFUL_STOP_TIMEOUT: std::time::Duration = std::time::Duration::from_mins(3);
 
 /// Max time `PauseDrain` waits for the harness to report
@@ -353,9 +351,7 @@ async fn run_meta_lock(
         // Grow one rebuild subgraph per stale agent into *this* boot DAG
         // (rooted on this `MetaLock`, so they build against the post-bump
         // lock), rather than fanning out child DAGs. The caller grows them
-        // with the graceful flavour: the per-agent drains overlap, so the
+        // with the graceful flavour.
-        // sweep's cost ceiling is one `GRACEFUL_STOP_TIMEOUT` in total, not
-        // one per agent.
         return Ok(fanout.unwrap_or_default());
     }
     let _progress = coord.meta_update_guard();