refactor(fe): trim impl-history and duplicated-docs prose from comments

Remove or shorten comment blocks that:
- explain where code used to live (impl history)
- duplicate rationale already in docs/web-ui.md
- contain speculative/future-work notes

Changes:
- tabs.js: drop "SYST3M tab used to render this" history; keep badge purpose
- schedules.js: drop "used to be a card layout" history + speculative nit note
- stream-worker.js: replace 10-line problem-statement with docs pointer
  (rationale already in docs/web-ui.md SSE multiplexing section); trim
  future-work parenthetical from subscription-tracking comment
- system-sections.css: drop extraction-history prose; keep what it does
- core.css: drop move-history prose; keep what it imports and why

Part of issue cleanup per operator feedback.

frontend/packages/dashboard/src/tabs.js

@@ -129,13 +129,8 @@ window.marked = marked;
     if (containersState.delete(ev.name)) renderContainersFromState();
   }
 
-  // Derived rebuild queue state — cold-loaded from
-  // `/api/state.rebuild_queue`, then mutated live by the
-  // `rebuild_queue_changed` snapshot event. The SYST3M tab that used to
-  // render this list moved to the standalone /core.html page; the
-  // dashboard keeps the STATE because it drives the "building…" /
-  // "meta-updating…" badges on the SW4RM agent cards (see
-  // inFlightOpsByAgent + docs/web-ui.md::Container row).
+  // Rebuild queue state — drives "building…" / "meta-updating…" badges on
+  // SW4RM agent cards (see inFlightOpsByAgent + docs/web-ui.md::Container row).
   let rebuildQueueState = [];
   function syncRebuildQueueFromSnapshot(s) {
     rebuildQueueState = (s.rebuild_queue || []).slice();