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/src/codexlens/watcher/events.py

@@ -0,0 +1,54 @@
+"""Event types for file watcher."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from enum import Enum
+from pathlib import Path
+from typing import List, Optional, Set
+
+
+class ChangeType(Enum):
+    """Type of file system change."""
+    CREATED = "created"
+    MODIFIED = "modified"
+    DELETED = "deleted"
+    MOVED = "moved"
+
+
+@dataclass
+class FileEvent:
+    """A file system change event."""
+    path: Path
+    change_type: ChangeType
+    timestamp: float
+    old_path: Optional[Path] = None  # For MOVED events
+
+
+@dataclass
+class WatcherConfig:
+    """Configuration for file watcher."""
+    debounce_ms: int = 1000
+    ignored_patterns: Set[str] = field(default_factory=lambda: {
+        ".git", ".venv", "venv", "node_modules",
+        "__pycache__", ".codexlens", ".idea", ".vscode",
+    })
+    languages: Optional[List[str]] = None  # None = all supported
+
+
+@dataclass
+class IndexResult:
+    """Result of processing file changes."""
+    files_indexed: int = 0
+    files_removed: int = 0
+    symbols_added: int = 0
+    errors: List[str] = field(default_factory=list)
+
+
+@dataclass
+class WatcherStats:
+    """Runtime statistics for watcher."""
+    files_watched: int = 0
+    events_processed: int = 0
+    last_event_time: Optional[float] = None
+    is_running: bool = False