feat(retrieval): pin model revisions and share encoder/reranker instances

Pin the Qwen embedding/reranker models to specific commit revisions (and
sentence-transformers to the exact version already in use) so index
builds are reproducible instead of floating on each model's main branch.

Also move the encoder and reranker instances to module-level singletons
shared by every Index, instead of one pair per instance, so a process
serving multiple repository roots only loads each model once.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

shared/retrieval/requirements.txt

@@ -1 +1 @@
-sentence-transformers>=5.4,<6
+sentence-transformers==5.7.0