feat(codex-lens): add unified reranker architecture and file watcher

Unified Reranker Architecture:
- Add BaseReranker ABC with factory pattern
- Implement 4 backends: ONNX (default), API, LiteLLM, Legacy
- Add .env configuration parsing for API credentials
- Migrate from sentence-transformers to optimum+onnxruntime

File Watcher Module:
- Add real-time file system monitoring with watchdog
- Implement IncrementalIndexer for single-file updates
- Add WatcherManager with signal handling and graceful shutdown
- Add 'codexlens watch' CLI command
- Event filtering, debouncing, and deduplication
- Thread-safe design with proper resource cleanup

Tests: 16 watcher tests + 5 reranker test files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

codex-lens/tests/test_hybrid_search_reranker_backend.py

@@ -0,0 +1,139 @@
+"""Tests for HybridSearchEngine reranker backend selection."""
+
+from __future__ import annotations
+
+import pytest
+
+from codexlens.config import Config
+from codexlens.search.hybrid_search import HybridSearchEngine
+
+
+def test_get_cross_encoder_reranker_uses_factory_backend_legacy(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path,
+) -> None:
+    calls: dict[str, object] = {}
+
+    def fake_check_reranker_available(backend: str):
+        calls["check_backend"] = backend
+        return True, None
+
+    sentinel = object()
+
+    def fake_get_reranker(*, backend: str, model_name=None, device=None, **kwargs):
+        calls["get_args"] = {
+            "backend": backend,
+            "model_name": model_name,
+            "device": device,
+            "kwargs": kwargs,
+        }
+        return sentinel
+
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.check_reranker_available",
+        fake_check_reranker_available,
+    )
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.get_reranker",
+        fake_get_reranker,
+    )
+
+    config = Config(
+        data_dir=tmp_path / "legacy",
+        enable_reranking=True,
+        enable_cross_encoder_rerank=True,
+        reranker_backend="legacy",
+        reranker_model="dummy-model",
+    )
+    engine = HybridSearchEngine(config=config)
+
+    reranker = engine._get_cross_encoder_reranker()
+    assert reranker is sentinel
+    assert calls["check_backend"] == "legacy"
+
+    get_args = calls["get_args"]
+    assert isinstance(get_args, dict)
+    assert get_args["backend"] == "legacy"
+    assert get_args["model_name"] == "dummy-model"
+    assert get_args["device"] is None
+
+
+def test_get_cross_encoder_reranker_uses_factory_backend_onnx_gpu_flag(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path,
+) -> None:
+    calls: dict[str, object] = {}
+
+    def fake_check_reranker_available(backend: str):
+        calls["check_backend"] = backend
+        return True, None
+
+    sentinel = object()
+
+    def fake_get_reranker(*, backend: str, model_name=None, device=None, **kwargs):
+        calls["get_args"] = {
+            "backend": backend,
+            "model_name": model_name,
+            "device": device,
+            "kwargs": kwargs,
+        }
+        return sentinel
+
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.check_reranker_available",
+        fake_check_reranker_available,
+    )
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.get_reranker",
+        fake_get_reranker,
+    )
+
+    config = Config(
+        data_dir=tmp_path / "onnx",
+        enable_reranking=True,
+        enable_cross_encoder_rerank=True,
+        reranker_backend="onnx",
+        embedding_use_gpu=False,
+    )
+    engine = HybridSearchEngine(config=config)
+
+    reranker = engine._get_cross_encoder_reranker()
+    assert reranker is sentinel
+    assert calls["check_backend"] == "onnx"
+
+    get_args = calls["get_args"]
+    assert isinstance(get_args, dict)
+    assert get_args["backend"] == "onnx"
+    assert get_args["model_name"] is None
+    assert get_args["device"] is None
+    assert get_args["kwargs"]["use_gpu"] is False
+
+
+def test_get_cross_encoder_reranker_returns_none_when_backend_unavailable(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path,
+) -> None:
+    def fake_check_reranker_available(backend: str):
+        return False, "missing deps"
+
+    def fake_get_reranker(*args, **kwargs):
+        raise AssertionError("get_reranker should not be called when backend is unavailable")
+
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.check_reranker_available",
+        fake_check_reranker_available,
+    )
+    monkeypatch.setattr(
+        "codexlens.semantic.reranker.get_reranker",
+        fake_get_reranker,
+    )
+
+    config = Config(
+        data_dir=tmp_path / "unavailable",
+        enable_reranking=True,
+        enable_cross_encoder_rerank=True,
+        reranker_backend="onnx",
+    )
+    engine = HybridSearchEngine(config=config)
+
+    assert engine._get_cross_encoder_reranker() is None