refactor: Unify examples interface with BaseRAGExample

- Create BaseRAGExample base class for all RAG examples
- Refactor 4 examples to use unified interface:
  - document_rag.py (replaces main_cli_example.py)
  - email_rag.py (replaces mail_reader_leann.py)
  - browser_rag.py (replaces google_history_reader_leann.py)
  - wechat_rag.py (replaces wechat_history_reader_leann.py)
- Maintain 100% parameter compatibility with original files
- Add interactive mode support for all examples
- Unify parameter names (--max-items replaces --max-emails/--max-entries)
- Update README.md with new examples usage
- Add PARAMETER_CONSISTENCY.md documenting all parameter mappings
- Keep main_cli_example.py for backward compatibility with migration notice

All default values, LeannBuilder parameters, and chunking settings
remain identical to ensure full compatibility with existing indexes.

examples/PARAMETER_CONSISTENCY.md

@@ -0,0 +1,64 @@
+# Parameter Consistency Guide
+
+This document ensures that the new unified interface maintains exact parameter compatibility with the original examples.
+
+## Parameter Mapping
+
+### Common Parameters (All Examples)
+
+| Parameter | Default Value | Notes |
+|-----------|--------------|-------|
+| `backend_name` | `"hnsw"` | All examples use HNSW backend |
+| `graph_degree` | `32` | Consistent across all |
+| `complexity` | `64` | Consistent across all |
+| `is_compact` | `True` | NOT `compact_index` |
+| `is_recompute` | `True` | NOT `use_recomputed_embeddings` |
+| `num_threads` | `1` | Force single-threaded mode |
+| `chunk_size` | `256` | Consistent across all |
+
+### Example-Specific Defaults
+
+#### document_rag.py (replaces main_cli_example.py)
+- `index_dir`: `"./test_doc_files"` (matches original)
+- `chunk_overlap`: `128` (matches original)
+- `embedding_model`: `"facebook/contriever"`
+- `embedding_mode`: `"sentence-transformers"`
+- No max limit by default
+
+#### email_rag.py (replaces mail_reader_leann.py)
+- `index_dir`: `"./mail_index"` (matches original)
+- `max_items`: `1000` (was `max_emails`)
+- `chunk_overlap`: `25` (matches original)
+- `embedding_model`: `"facebook/contriever"`
+- NO `embedding_mode` parameter in LeannBuilder (original doesn't have it)
+
+#### browser_rag.py (replaces google_history_reader_leann.py)
+- `index_dir`: `"./google_history_index"` (matches original)
+- `max_items`: `1000` (was `max_entries`)
+- `chunk_overlap`: `25` (primary value in original)
+- `embedding_model`: `"facebook/contriever"`
+- `embedding_mode`: `"sentence-transformers"`
+
+#### wechat_rag.py (replaces wechat_history_reader_leann.py)
+- `index_dir`: `"./wechat_history_magic_test_11Debug_new"` (matches original)
+- `max_items`: `50` (was `max_entries`, much lower default)
+- `chunk_overlap`: `25` (matches original)
+- `embedding_model`: `"Qwen/Qwen3-Embedding-0.6B"` (special model for Chinese)
+- NO `embedding_mode` parameter in LeannBuilder (original doesn't have it)
+
+## Implementation Notes
+
+1. **Parameter Names**: The original files use `is_compact` and `is_recompute`, not the newer names.
+
+2. **Chunk Overlap**: Most examples use `25` except for documents which uses `128`.
+
+3. **Embedding Mode**: Only `google_history_reader_leann.py` and `main_cli_example.py` have this parameter.
+
+4. **Max Items**: Each example has different defaults:
+   - Email/Browser: 1000
+   - WeChat: 50
+   - Documents: unlimited
+
+5. **Special Cases**:
+   - WeChat uses a specific Chinese embedding model
+   - Email reader includes HTML processing option