Move the changelog entry to changelog.d and clear review leftovers

The entry now lives in changelog.d/1471-context-panel-cleanup.md with
credit, as CONTRIBUTING asks, and CHANGELOG.md is back to main.

CURRENT-STATE no longer lists a compact Radio power control and scopes
the readability description to what Cyber keeps. The responsive glow
comment no longer cites the removed Radio popover, and qa-radio's
pick-buffer comment is back above settleRadioFrames.

Four test fixtures stop stubbing the removed launcher sync, and
PanelChrome drops the _syncRailPanelCollapseButton pass-through, wiring
the layout controller straight to _syncPanelCollapseButton.

src/ui/styles/responsive.css

@@ -205,7 +205,7 @@
      under the expanded panel plus a vertical scrollbar that scrolls nothing
      but glow. Pin the glow to the panel box here — its blur is ink overflow,
      which never scrolls — instead of clipping the rail, which would also clip
-     legitimate content such as the Context radio popover. */
+     legitimate content that extends past a panel's edge. */
   #left-panel-stack .panel-glow,
   #right-context-rail .panel-glow {
     inset: 0;