Sync notes through GPX, Journal, and Planner roundtrip

Full notes lifecycle:
- GPX: description field in GpxData, <metadata><desc> in generate/parse
- Export: Plan GPX and Save to Journal include notes as description
- Journal: updateRoute extracts description from GPX, stores on route
- Reimport: Edit in Planner passes notes via URL params → Yjs Y.Text
- Drop import: GPX with <desc> restores notes in session

Spec updated: session-notes gains GPX export, Journal sync, and
reimport requirements.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/planner/app/components/ExportButton.tsx

@@ -69,7 +69,8 @@ export function ExportButton({ yjs }: { yjs: YjsState }) {
     const tracks = getTracks(yjs);
     const waypoints = getWaypoints(yjs);
     const noGoAreas = getNoGoAreas(yjs);
-    const gpx = generateGpx({ name: "trails.cool route", waypoints, tracks, noGoAreas });
+    const notes = yjs.notes.toString() || undefined;
+    const gpx = generateGpx({ name: "trails.cool route", description: notes, waypoints, tracks, noGoAreas });
     download(gpx, "route-plan.gpx");
     setOpen(false);
   }, [yjs]);