chore: switch active trip to italy-2026-demo, remove japan-korea-2026

- active_trip: italy-2026-demo in site.yaml (was japan-korea-2026)
- post-form.md parent updated to /trips/italy-2026-demo/dailies
- Remove japan-korea-2026 trip pages (no real trip exists)
- Remove stale old italy-2026-demo entries/stories/GPX from git tracking
  (these were leftover from before the demo-source approach)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

pages/02.post/post-form.md

@@ -6,7 +6,7 @@ access:
 
 # Keep in sync with active_trip in user/config/site.yaml
 pageconfig:
-    parent: '/trips/japan-korea-2026/dailies'
+    parent: '/trips/italy-2026-demo/dailies'
     slug_field: 'date,title'
     overwrite_mode: false