Add weeknotes system with /weeknotes command

- Create weeknotes archetype for manual post creation
- Add /weeknotes slash command that prompts through each section
  interactively (shipped, read, played, cooked, noticed, thinking, next)
- Add /weeknotes landing page explaining the concept
- Update CLAUDE.md with weeknotes documentation and voice guidelines

Weeknotes are brief weekly reflections—lighter than full posts, more
personal journal than polished article.

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

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

archetypes/weeknotes.md

@@ -0,0 +1,37 @@
+---
+title: "Weeknotes: {{ dateFormat "January 2, 2006" .Date }}"
+date: {{ .Date }}
+draft: true
+tags:
+  - weeknotes
+---
+
+_[One sentence capturing the week's vibe]_
+
+## Shipped
+
+-
+
+## Read
+
+-
+
+## Played
+
+-
+
+## Cooked
+
+-
+
+## Noticed
+
+-
+
+## Thinking About
+
+-
+
+## What's Next
+
+-