feat: add clean command to remove temp files, logs, and cached data

Adds `unarr clean` with interactive confirmation, --dry-run, --yes,
and --all flags. Safely skips recent usenet resume files (<7 days) to
preserve download progress. Includes platform-specific PID detection
(Unix signal 0 / Windows heartbeat heuristic), CleanableBytes callback
for future heartbeat reporting, and uses shared ui.FormatBytes.

internal/agent/process_unix.go

@@ -0,0 +1,11 @@
+//go:build !windows
+
+package agent
+
+import "syscall"
+
+// IsProcessAlive checks if a process with the given PID is running.
+// On Unix, sends signal 0 which checks existence without affecting the process.
+func IsProcessAlive(pid int) bool {
+	return syscall.Kill(pid, 0) == nil
+}