Rewrite in Go: static binaries, zero runtime dependencies

Replace Node.js + Python codebase with three Go binaries:
- claude-statusline: CLI status bar for Claude Code
- claude-fetcher: standalone cron job for API usage
- claude-widget: system tray icon (fyne-io/systray + fogleman/gg)

All CGO-free for trivial cross-compilation. Add nfpm .deb packaging
with autostart and cron. CI pipeline produces Linux + Windows binaries,
.deb, .tar.gz, and .zip release assets.

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

packaging/linux/claude-widget.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Type=Application
+Name=Claude Usage Widget
+Comment=System tray widget showing Claude API token usage
+Exec=/usr/bin/claude-widget
+Terminal=false
+Categories=Utility;
+StartupNotify=false
+X-GNOME-Autostart-enabled=true