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>

internal/fetcher/cache.go

@@ -0,0 +1,85 @@
+package fetcher
+
+import (
+	"encoding/json"
+	"os"
+	"runtime"
+	"time"
+)
+
+// CacheData represents the JSON structure written to the cache file.
+// It mirrors the raw API response (five_hour, seven_day) plus error fields.
+type CacheData struct {
+	FiveHour *UsageWindow `json:"five_hour,omitempty"`
+	SevenDay *UsageWindow `json:"seven_day,omitempty"`
+	Error    string       `json:"_error,omitempty"`
+	Status   int          `json:"_status,omitempty"`
+	Message  string       `json:"_message,omitempty"`
+}
+
+// UsageWindow represents a single usage window from the API.
+type UsageWindow struct {
+	Utilization float64 `json:"utilization"`
+	ResetsAt    string  `json:"resets_at"`
+}
+
+// CachePath returns the cache file path.
+func CachePath() string {
+	if v := os.Getenv("CLAUDE_USAGE_CACHE"); v != "" {
+		return v
+	}
+	if runtime.GOOS == "windows" {
+		if tmp := os.Getenv("TEMP"); tmp != "" {
+			return tmp + `\claude_usage.json`
+		}
+		return os.TempDir() + `\claude_usage.json`
+	}
+	return "/tmp/claude_usage.json"
+}
+
+// WriteCache writes data to the cache file as JSON.
+func WriteCache(data *CacheData) error {
+	b, err := json.MarshalIndent(data, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(CachePath(), b, 0o644)
+}
+
+// ReadCache reads and parses the cache file. Returns data and file age.
+// Returns nil if file doesn't exist or can't be parsed.
+func ReadCache() (*CacheData, time.Duration) {
+	path := CachePath()
+	info, err := os.Stat(path)
+	if err != nil {
+		return nil, 0
+	}
+	age := time.Since(info.ModTime())
+
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return nil, 0
+	}
+
+	var data CacheData
+	if err := json.Unmarshal(raw, &data); err != nil {
+		return nil, 0
+	}
+	return &data, age
+}
+
+// ReadCacheIfFresh reads cache only if it's younger than maxAge.
+// Error caches are always returned regardless of age.
+func ReadCacheIfFresh(maxAge time.Duration) *CacheData {
+	data, age := ReadCache()
+	if data == nil {
+		return nil
+	}
+	if data.Error != "" {
+		return data
+	}
+	if age > maxAge {
+		return nil
+	}
+	return data
+}