Add llmProvider, llmModel, and llmApiKeyEnv to the plugin config schema.
These allow users to choose which LLM Hindsight uses directly from
openclaw.json config without needing HINDSIGHT_API_LLM_* env vars.
Priority order (highest to lowest):
1. HINDSIGHT_API_LLM_PROVIDER env var (unchanged)
2. Plugin config llmProvider/llmModel (NEW)
3. Auto-detect from provider env vars (unchanged)
Backward compatible: no config = same behavior as before.
