rename: cosmic-brain → claude-obsidian

Updates all references across README, CLAUDE.md, ATTRIBUTION.md,
WIKI.md, docs/install-guide.md, wiki/getting-started.md,
plugin.json, marketplace.json, commands/wiki.md, setup-vault.sh,
wiki pages, and renames wiki/meta/cosmic-brain-cover.gif.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

wiki/getting-started.md

@@ -14,7 +14,7 @@ related:
   - "[[Wiki Map]]"
 ---
 
-# Getting Started with cosmic-brain
+# Getting Started with claude-obsidian
 
 Welcome. This vault is your compounding knowledge base — a persistent second brain built with Claude and Obsidian.