v0.5.5 - ACM-9: System-reminder injection

ACM-9: Inject system-reminder with wall-clock time and context limit

Implements experimental.chat.system.transform to inject a <system-reminder> block on every turn containing:

• Current wall-clock time (locale string with timezone)
• Context token limit (from OPENCODE_CONTEXT_STATUS_LIMIT env or model context window)

Skips injection if OpenCode has already injected its own time-containing system-reminder to avoid duplication.