Improve post slug: date + time + title

Slug format: 2026-06-18-1430-arrived-in-tokyo
- Eliminates one-post-per-day collision (BUG TC-P.10)
- URL is stable after creation (time baked in, title change doesn't affect it)
- regex_replace strips non-alphanumeric chars; trim('-') cleans edges

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pages/02.post/post-form.md

@@ -86,7 +86,7 @@ form:
         -
             add-page-by-form:
                 parent: '/tracker'
-                slug: "{{ form.value.date|date('Y-m-d') }}"
+                slug: "{{ form.value.date|date('Y-m-d-Hi') }}-{{ form.value.title|lower|regex_replace('/[^a-z0-9]+/', '-')|trim('-') }}"
                 template: 'entry'
                 frontmatter:
                     title: '{{ form.value.title }}'