Add automated installer with OS detection and curl pipe support

- install.sh: One-command installer with macOS/Ubuntu/Arch detection
  - Supports curl pipe (remote) and local execution
  - Interactive sudo prompt for Linux, user-level by default
  - Idempotent rc file edits with marker blocks
  - Backs up existing configs before overwriting
  - Installs: fonts, tmux, starship, fetch scripts, iTerm2 colors
  - --uninstall flag restores backups and removes all changes
- Update .gitignore to include fonts/ directory
- Update README.md with install instructions
- Update TERMINAL-SETUP.md to reference automated installer

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,5 +1,5 @@
-# Font files — included in private mirror only
-# fonts/
+# Font files — download separately from nerdfonts.com
+fonts/
 
 # macOS
 .DS_Store