Add grep search functionality - Issue #86 (#87)

* Add grep search functionality to LeannSearcher

- Add use_grep parameter to search method
- Implement grep-based search on .jsonl files
- Add fallback Python regex search
- Support same SearchResult format as semantic search

Addresses issue #86

* fix: resolve linting errors

* docs: add grep search example

* docs: add grep search to README examples

* refactor: remove regex fallback, move grep example to features section

* docs: add grep search to Advanced Features with comprehensive guide

README.md

@@ -656,6 +656,19 @@ results = searcher.search(
 
 📖 **[Complete Metadata filtering guide →](docs/metadata_filtering.md)**
 
+### 🔍 Grep Search
+
+For exact text matching instead of semantic search, use the `use_grep` parameter:
+
+```python
+# Exact text search
+results = searcher.search("banana‑crocodile", use_grep=True, top_k=1)
+```
+
+**Use cases**: Finding specific code patterns, error messages, function names, or exact phrases where semantic similarity isn't needed.
+
+📖 **[Complete grep search guide →](docs/grep_search.md)**
+
 ## 🏗️ Architecture & How It Works
 
 <p align="center">