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/schedules.js

@@ -683,11 +683,7 @@ function renderScheduleRow(s, agents) {
 
   tr.append(el('td', { class: 'meta' }, s.owner));
 
-  // Body cell — truncates with ellipsis; full body + description (if
-  // any) on hover. Description used to be a separate visible block on
-  // the card layout; the table compresses it into the title to keep
-  // row height tight. Mara nit-flag candidate if she actually wants
-  // it visible in-table.
+  // Body cell — truncates with ellipsis; full body + description on hover.
   const bodyCell = el('td', { class: 'schedules-table-body-cell' });
   const bodyText = s.body || '';
   bodyCell.title = (s.description ? s.description + '\n\n' : '') + bodyText;