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/stream-worker.js

@@ -1,16 +1,6 @@
 // SharedWorker that holds ONE EventSource per stream URL and fans
 // every server-sent event out to every connected tab via MessagePort.
-//
-// Problem this solves: every dashboard / agent tab opens its own
-// `EventSource('/api/dashboard/stream')`. Browsers cap concurrent
-// connections per host (~6), and Firefox throttles / disconnects
-// background tabs when many are open. The result: tabs silently
-// drop the SSE, fall behind, and only catch up on focus.
-//
-// Centralising the connection in a SharedWorker means N tabs share
-// ONE backend EventSource regardless of focus state — way under the
-// per-host cap, immune to per-tab throttling, and the worker survives
-// any individual tab being suspended.
+// Design rationale: docs/web-ui.md (SSE multiplexing paragraph).
 //
 // Wire protocol (port.postMessage payloads):
 //
@@ -42,8 +32,7 @@
 //                                       signal to the client).
 //
 // Subscriptions are tracked per (port, url): a single port can
-// subscribe to multiple URLs (today only one is in use but the shape
-// stays open for the per-agent /events/stream multiplexing follow-up).
+// subscribe to multiple URLs (only one is in use today).
 // Unsubscribing the last port for a URL closes the EventSource so we
 // don't keep idle streams open.