Fix: skip Obsidian/Claude Code install when already present

[Levis44]

• Avoid redundant installs by treating Obsidian as “installed” if it exists via Homebrew or as /Applications/Obsidian.app.
• Harden install flow by handling failures:
  • Obsidian cask install failure now prints the official download link.
  • Claude Code install script failure now prints a manual npm install -g @anthropic-ai/claude-code fallback.
• Post-install check now matches the same Obsidian detection logic (Homebrew cask OR /Applications/Obsidian.app).