cache-timer: prompt-cache countdown & refresh for OpenCode

A TUI plugin for OpenCode that displays a live countdown timer until the prompt-cache expires, letting you know when your cache is hot, warning, cold, or busy-cold. It provides one-click buttons to refresh the cache (keeps it hot) or fork a new session (when cache is cold), reducing costly cold-write operations. Version 1.2.0, actively maintained. Differentiators: pre-emptive cache saving, smart cost-based button logic, minimal configuration.