refactor(embedder-params): drop `dimensions` knob; pass indexing kwargs into LiteLLM ctor (#151)

- Remove `dimensions` from the litellm whitelist in `_ACCEPTED_KWARGS`.
  Output dimension must be identical for indexing and query for vectors to
  be comparable, so it's a model-wide setting, not a per-side knob —
  exposing it under `indexing_params` / `query_params` invited
  misconfiguration. Updated comment template, README, design doc, and
  testing plan accordingly.
- Plumb `indexing_params` into `create_embedder` and pass them as
  constructor kwargs to `PacedLiteLLMEmbedder`. The values land in
  `self._kwargs` and become defaults forwarded into every
  `litellm.aembedding` call — including paths that don't go through the
  `INDEXING_EMBED_PARAMS` context var (e.g. the dim probe in `_get_dim`).
  Per-call overrides (`query_params` spread at query time) still win
  because `_embed` overlays kwargs on top of `self._kwargs`. Sentence-
  transformers ignores `indexing_params` (its constructor doesn't accept
  arbitrary kwargs; `prompt_name` is per-call only).