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/postinstall.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+set -e
+
+CONFIG_DIR="${HOME}/.config/claude-statusline"
+mkdir -p "$CONFIG_DIR"
+
+if [ ! -f "$CONFIG_DIR/session-key" ]; then
+    touch "$CONFIG_DIR/session-key"
+    chmod 600 "$CONFIG_DIR/session-key"
+    echo ""
+    echo "=== Claude Statusline ==="
+    echo "Paste your claude.ai session key into:"
+    echo "  $CONFIG_DIR/session-key"
+    echo ""
+    echo "To get your session key:"
+    echo "  1. Open claude.ai in your browser"
+    echo "  2. Open DevTools (F12) → Application → Cookies"
+    echo "  3. Copy the 'sessionKey' cookie value"
+    echo "========================="
+    echo ""
+fi