chore: remove _notes and .obsidian from git tracking

- Untrack _notes/ vault and .obsidian/ config
- Update .gitignore to exclude both permanently

.gitignore

@@ -53,19 +53,12 @@ web/assets/login-bg.mp4
 # large binary assets — keep local only, not tracked in git
 assets/login-bg.mp4
 
-# Obsidian — exclude personal workspace state, keep plugin/vault config
-.obsidian/workspace.json
-.obsidian/workspace-mobile.json
-.obsidian/cache
-
-# Claude Code / MCP / vibe-coding tool artifacts
+# Claude Code / MCP / vibe-coding tool artifacts — keep local only
 .claude/
 .mcp.json
 CLAUDE.md
+_notes/
+.obsidian/
 hero_section_improvements.csv
 security_audit_report.csv
 feature_gap_analysis.csv
-
-# Obsidian workspace state in nested vaults
-**/.obsidian/workspace.json
-**/.obsidian/workspace-mobile.json