feat(retrieval): add pluggable cross-encoder reranker

[vcvyg]

Summary

• add a batched CrossEncoderReranker that implements the existing reranker protocol and lazily loads the optional Sentence Transformers dependency
• make heuristic, llm, and cross_encoder selectable through retrieval settings while preserving the existing LLM reranking path
• document configuration and add a reproducible recall/latency benchmark with stub-backed unit coverage

Validation

• uv run ruff check src/contextseek tests/unit_tests scripts/benchmark_rerankers.py
• uv run pytest tests/ -q --ignore=tests/unit_tests/test_powermem_plug.py (451 passed, 8 skipped)
• uv run --extra rerank python scripts/benchmark_rerankers.py --device cpu --rounds 5

Closes #49

[vcvyg]

#49

[Copilot]

Pull request overview

This PR introduces a pluggable retrieval reranker option by adding a lazily-loaded Sentence Transformers cross-encoder implementation, wiring it into settings-driven configuration, and documenting/benchmarking the new path.

Changes:

• Added CrossEncoderReranker (batched scoring, lazy dependency import) and exposed it via retrieval module exports.
• Added reranker_mode=cross_encoder configuration via RetrievalSettings + factory wiring, and injected the reranker into ContextSeek retrieval.
• Added unit tests, documentation updates, and a small benchmark script for recall/latency comparison.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
uv.lock	Adds sentence-transformers and updates lock resolution to support the new optional extra.
pyproject.toml	Introduces rerank extra and includes sentence-transformers in all.
src/contextseek/retrieval/components.py	Implements CrossEncoderReranker with lazy model loading and batched prediction.
src/contextseek/retrieval/init.py	Re-exports CrossEncoderReranker from retrieval package.
src/contextseek/config/settings.py	Adds cross-encoder configuration fields to RetrievalSettings.
src/contextseek/config/factory.py	Adds build_reranker() to construct non-LLM rerankers based on settings.
src/contextseek/config/init.py	Exports build_reranker for external configuration assembly.
src/contextseek/config/strategies.py	Updates comment/documentation for reranker_mode in RetrievalStrategy.
src/contextseek/client/contextseek.py	Injects configured reranker into retrieval; preserves existing LLM rerank behavior.
tests/unit_tests/test_settings.py	Adds settings/factory coverage for cross-encoder reranker selection and validation.
tests/unit_tests/retrieval/test_reranker_features.py	Adds stub-backed unit tests for cross-encoder reranking behavior and fallback.
scripts/benchmark_rerankers.py	Adds reproducible benchmark script for heuristic vs cross-encoder recall/latency.
docs/en/reference/settings.md	Documents new RETRIEVAL_CROSS_ENCODER_* settings and mode.
docs/zh/reference/settings.md	Chinese settings reference updates for cross-encoder mode/settings.
docs/en/concepts/retrieval-model.md	Documents cross-encoder reranker usage and configuration.
docs/en/concepts/reranker-benchmark.md	Adds benchmark note and reference results table.
.env.example	Adds example env vars for cross-encoder reranker configuration.

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