chore(index): remove the unused vector index, the ousast index subcommand and the [embeddings] section

Legacy inventory 2, item R1. The vector half of index.py (VectorIndex, VectorRecord, RetrievalHit,
RetrievalPackage, the EmbeddingClient protocol, index_reuse_key, build/write/read/query_vector_index,
build_retrieval_package and their helpers) had no importer in src; `ousast index` wrote index/chunks.json
that nothing read, and was the only reader of `[embeddings]`. index.py keeps CodeChunk and
chunk_text_namespace, which skills.py imports; chunk_file/build_code_chunks go with their only caller.

`[embeddings] model` and `[embeddings] store` join RETIRED_KEYS (RetiredConfigError naming the reason),
the same way the [variants] keys were retired; OPENROUTER_EMBEDDING_MODEL is no longer read, so the
README and .env.example drop it (`ousast learn memory embed --model` names the model). test_index.py
now tests the chunker only; test_config.py covers the two retired keys and no longer asserts the
removed field. The module audit record is regenerated (counts unchanged: 151 modules, 0 orphaned;
the index module's importer list shrinks to skills). RELEASE_NOTES Unreleased carries the line.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.env.example

@@ -6,9 +6,8 @@
 DEEPSEEK_API_KEY=
 # DEEPSEEK_BASE_URL=https://api.deepseek.com
 
-# Embeddings only: OpenAI text-embedding-3-small through OpenRouter.
+# Embeddings only (`ousast learn memory embed`, default openai/text-embedding-3-small through OpenRouter).
 OPENROUTER_API_KEY=
-OPENROUTER_EMBEDDING_MODEL=openai/text-embedding-3-small
 
 # Plane memory store. Default is a local FileStore under ~/ousast-results/plane/memory.
 # For an S3-compatible server (RustFS is the tested one): OUSAST_MEMORY=s3://<bucket>[/<prefix>], or