docs(#3034): give the brace one home instead of five

Measured after mara's "2/3 of this is docs, most of it duplicated": 243 of 380
added .rs lines were comments. The brace rationale was written out in full in
`model.rs`, the `templates.rs` module header, `quiesce`, `rebuild_subtree` and
`docs/coordinator.md` — five copies of one argument, which is why four docs
needed correcting earlier in this branch. Correcting every copy preserves the
thing that made them go stale.

`docs/coordinator.md` (_Braces_) is now the single home. The rest state what a
node *is* and point there. Also drops the per-operation DAG diagram from the
`templates.rs` header, which the same doc already carries, and cuts
`rebuild_subtree`'s node-by-node walkthrough down to the three choices a reader
would otherwise undo — the code below it is the source of truth for the shape.

Comments -59 lines, no behaviour change, 317 tests unchanged.

hive-c0re/src/job_queue/exec.rs

@@ -130,11 +130,8 @@ pub(super) async fn run_node(
         // reach `Finishing` so the nodes under it start.
         // - `Dag`: pure grouping container. The DAG's terminal side effect, if
         //   any, is its own tail node in the graph.
-        // - `DeployWindow`: pure resource holder — the meta window, agent lease
-        //   and build slot it declares stay held until its subtree settles.
-        // - `AgentWindow`: pure resource holder for one agent's rebuild — holds
-        //   the lease so the build and the graceful-stop window can run as
-        //   siblings rather than one nested under the other.
+        // - `DeployWindow` / `AgentWindow`: pure resource holders (braces) —
+        //   what they declare stays held until their subtree settles.
         NodeKind::Dag { .. } | NodeKind::DeployWindow { .. } | NodeKind::AgentWindow { .. } => {
             Ok(())
         }