fix: audit cleanup — untrack personal files, fix stale comment, add audit plan

- Untrack Cosmic Brain Clean.gif + Cosmic Brain Cover.png (personal assets)
- Untrack Welcome.md (Obsidian personal file)
- Add gitignore rules for Welcome.md and Cosmic Brain*.gif/png
- Fix vault-colors.css comment: cosmic-brain → claude-obsidian
- Add docs/superpowers/plans/2026-04-07-full-repo-audit.md

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

.gitignore

@@ -36,9 +36,12 @@ node_modules/
 # Personal files dropped into vault root by Obsidian (don't commit these)
 WIKI*.md
 PROMPT.md
+Welcome.md
 *.tmp.*
 Obsidian vault*
 Untitled.canvas
+Cosmic Brain*.gif
+Cosmic Brain*.png
 Untitled *.canvas
 Untitled.base
 *.base