Add a0_reranker: cross-encoder reranking of memory recall

[King0James0]

Adds a0_reranker — cross-encoder reranking of Agent Zero's memory recall.

• Wraps the memory similarity search: pulls a larger candidate pool, reranks it against the user's own reranker endpoint, returns the best matches first.
• Auto-detects the server's API with a correctness probe (standard rerank — Cohere/Jina/Infinity/TEI/vLLM /v1/rerank — or a vLLM-served Qwen3-Reranker via client-templated /score, which answers /v1/rerank with meaningless scores unless the judge template is applied client-side).
• Fail-safe: any reranker error falls back to the original recall order; no endpoint configured = inert. Shadow mode + a Test-connection button to verify before committing.
• Zero dependencies beyond Agent Zero (pure stdlib). Verified on v1.19 through v2.2. MIT.

Repo: https://github.com/King0James0/a0-reranker (Release v1.0.2 with the installable zip).