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

benchmarks/measurements/2026-09-08-module-audit.json

@@ -338,7 +338,6 @@
       "classification": "load_bearing",
       "reason": "reachable from gate, map_gate, pair_gate, cli",
       "importers": [
-        "cli",
         "skills"
       ]
     },
@@ -530,6 +529,7 @@
       "importers": [
         "learn.compile",
         "learn.evaluate",
+        "learn.examples",
         "learn.features",
         "learn.leaks",
         "learn.program",
@@ -1119,7 +1119,6 @@
         "hunter",
         "hunter_tools",
         "improve.evolve",
-        "index",
         "learn.features",
         "learn.labels",
         "learn.roles",