fix: audit cleanup — rename remaining Cosmic Brain references

- workspace.json: cosmic-brain-cover.gif → claude-obsidian-gif-cover-16x9.gif
- welcome.canvas: title node "Cosmic Brain" → "Claude Obsidian"

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

.obsidian/workspace.json

@@ -237,7 +237,7 @@
     "Untitled.base",
     "docs/install-guide.md",
     "README.md.tmp.224546.1775561616026",
-    "wiki/meta/cosmic-brain-cover.gif",
+    "wiki/meta/claude-obsidian-gif-cover-16x9.gif",
     "README.md.tmp.224546.1775561304170",
     "wiki/meta/cover.gif",
     "wiki/concepts/_index.md",