Add weeknotes system with first entry

- Add weeknotes landing page with auto-listing of all entries
- Create first weeknote: December 20–27, 2025
- Add Weeknotes to secondary menu alongside Now/Then/Future
- Create custom layout to display all weeknotes chronologically
- Update weeknotes command to use date ranges and current time
- Credit Genehack for weeknotes inspiration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

config.toml

@@ -97,6 +97,11 @@ theme = "m10c"
     name = "Future"
     url = "/upcoming/"
     weight = 3
+  [[menu.secondary]]
+    identifier = "weeknotes"
+    name = "Weeknotes"
+    url = "/weeknotes/"
+    weight = 4
   [[menu.tertiary]]
     identifier = "mytweets"
     name = "Local Tweet Archive"