Document Astro design decisions in CLAUDE.md; fix branch name

Document Astro design decisions in CLAUDE.md; fix branch name

Adds a 'Astro-Implementierung: Design- und Architektur-Entscheidungen'
section that records all non-trivial decisions made during the migration:
- CSS pre-compilation via separate sass step (not Astro built-in)
- Split taxonomy URL structure matching Hugo's output
- BlogNav as an explicit TypeScript state machine (vs. Hugo's recursive
  template partial)
- URL compatibility via frontmatter 'url' field
- Word count / reading time via inline string manipulation
- Summary extraction from raw Markdown body
- .html suffix routing via double-extension file naming
- Standalone pages without navigation (documented as open issue)

Also corrects the branch name in Arbeitsregeln from content--via-hugo
to content--via-astro.

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