docs: scrub dangling references to deleted roadmap docs

The ROADMAP/ROADMAP-PINCHFLAT docs were removed; this drops the now-dead
"see ROADMAP-PINCHFLAT.md" breadcrumbs (and one "the roadmap" mention)
from source comments, preserving the surrounding Pinchflat-style context.
Comment-only; typecheck (node+web) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/renderer/src/components/SourceRetention.tsx

@@ -42,7 +42,7 @@ const useStyles = makeStyles({
  *     target quality (from its profile or the global default).
  *
  * The policy inputs are ephemeral (per-open) — this is an on-demand tidy, not a
- * persisted schedule, matching the roadmap's "niche for a desktop app" scope.
+ * persisted schedule, keeping retention to a desktop-app-sized scope.
  */
 export function SourceRetention({ sourceId }: { sourceId: string }): React.JSX.Element {
   const styles = useStyles()