feat(cli): add anonymous telemetry via PostHog

Tracks cli_command events (command name, non-PII properties) and
cli_error events (error message, stack). Anonymous machine ID via
SHA-256 hash. Opt-out via PAYKIT_TELEMETRY_DISABLED=1, DO_NOT_TRACK=1,
or `paykitjs telemetry disable`. First-run notice shown once.