[ qingfeng/2020117 ] fix: session_start fallback when skill has no pricing.sats_per_minute

Provider now falls back to SATS_PER_MINUTE env var, customer's
proposed rate, or default 10 when skill.pricing is not set.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/qingfeng/2020117/commit/b61d98642551940db4864bd4247e94ea186a925e