dashboard --sixel getEnv isolation

Trap: setting process.env.SENTRY_DASHBOARD_SIXEL = "1" in dashboard view looks like a simple per-command opt-in. Fix: carry the flag in dashboard invocation data/context and consult the environment registry where needed. SDK invocations replace getEnv() with isolated environments, so direct process.env writes neither enable SDK rendering nor stay contained; in CLI mode, never call setEnv() and never leak Sixel into later same-process commands.