docs(models): document litellm-sdk embeddings provider

[r266-tech]

Summary

litellm-sdk is a real, functional embeddings provider (registered via embeddings_litellm_sdk_* config in hindsight-api-slim/hindsight_api/config.py, instantiated by the LiteLLMSDKEmbeddings class at embeddings.py:753, and listed in the unknown-provider error message: "Supported: 'local', 'tei', 'openai', 'cohere', 'google', 'litellm', 'litellm-sdk'" at embeddings.py:1209). It also gets continued investment — #1320 (merged a few hours ago) wired allowed_openai_params=["dimensions"] so HINDSIGHT_API_EMBEDDINGS_LITELLM_SDK_OUTPUT_DIMENSIONS actually works for OpenAI-compatible custom models.

But the embedding providers table at docs/developer/models.mdx only lists six providers (local / openai / cohere / google / tei / litellm) — litellm-sdk is missing. The cross-encoder providers table on the same page already documents it (| litellm-sdk | LiteLLM SDK (direct API, no proxy) | Multi-provider, simpler setup |), so users reading the embeddings section can't tell the SDK path is supported.

What's in the PR

• hindsight-docs/docs/developer/models.mdx (+5 LOC): one new row in the embedding providers table mirroring the cross-encoder description; one new # LiteLLM SDK (direct, no proxy) stanza in the Configuration Examples block (mirrors the existing # LiteLLM proxy stanza style).
• skills/hindsight-docs/references/developer/models.md (+5 LOC): byte-for-byte mirror.

Source of truth

• Provider name: LiteLLMSDKEmbeddings.provider_name returns "litellm-sdk" (embeddings.py:802); factory dispatches provider == "litellm-sdk" (embeddings.py:1173).
• Env var names: HINDSIGHT_API_EMBEDDINGS_LITELLM_SDK_API_KEY and HINDSIGHT_API_EMBEDDINGS_LITELLM_SDK_MODEL defined at config.py:223-224. API key is required (factory raises ValueError if unset, embeddings.py:1175-1178).
• Example model openai/text-embedding-3-small follows the LiteLLM provider-prefix convention used throughout the existing docs (and is the path fix(embeddings): add allowed_openai_params for OpenAI-compatible embedding dimensions #1320 just made fully functional).

Pure docs, no behavior change.

[nicoloboschi]

LGTM