feat: add documentation, Streamlit GUI, and full source tree

- Rewrite README.md with project overview, quick-start, and CLI summary
- Add docs/CLI-REFERENCE.md with full flag reference and 8 recipe sections
- Add docs/DEVELOPER.md with architecture, data flow, and extension guides
- Rewrite src/core/__init__.py with public API exports and module docstring
- Add Streamlit GUI (src/gui/) with file upload, advanced options, interactive
  match group review with side-by-side diff, and download buttons
- Add .gitignore, requirements.txt, all source code, tests, and sample data
- Add streamlit to requirements.txt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/__init__.py

@@ -0,0 +1,93 @@
+"""DataTools deduplication engine.
+
+Public API
+----------
+Core:
+    deduplicate(df, ...) -> DeduplicationResult
+    build_default_strategies(df) -> list[MatchStrategy]
+
+Types:
+    Algorithm, SurvivorRule, ColumnMatchStrategy, MatchStrategy
+    MatchResult, DeduplicationResult
+
+Normalizers:
+    get_normalizer(type) -> Callable
+    NormalizerType
+    normalize_email, normalize_phone, normalize_name,
+    normalize_address, normalize_string
+
+I/O:
+    read_file(path, ...) -> DataFrame
+    write_file(df, path, ...)
+    list_sheets(path) -> list[str]
+    detect_encoding, detect_delimiter, detect_header_row
+
+Configuration:
+    DeduplicationConfig.from_file(path) -> DeduplicationConfig
+    DeduplicationConfig.to_file(path)
+"""
+
+from .dedup import (
+    Algorithm,
+    ColumnMatchStrategy,
+    DeduplicationResult,
+    MatchResult,
+    MatchStrategy,
+    SurvivorRule,
+    build_default_strategies,
+    deduplicate,
+)
+from .normalizers import (
+    NormalizerType,
+    get_normalizer,
+    normalize_address,
+    normalize_email,
+    normalize_name,
+    normalize_phone,
+    normalize_string,
+)
+from .io import (
+    detect_delimiter,
+    detect_encoding,
+    detect_header_row,
+    list_sheets,
+    read_file,
+    write_file,
+)
+from .config import (
+    ColumnStrategyConfig,
+    DeduplicationConfig,
+    StrategyConfig,
+)
+
+__all__ = [
+    # Core
+    "deduplicate",
+    "build_default_strategies",
+    # Types
+    "Algorithm",
+    "SurvivorRule",
+    "ColumnMatchStrategy",
+    "MatchStrategy",
+    "MatchResult",
+    "DeduplicationResult",
+    # Normalizers
+    "NormalizerType",
+    "get_normalizer",
+    "normalize_email",
+    "normalize_phone",
+    "normalize_name",
+    "normalize_address",
+    "normalize_string",
+    # I/O
+    "read_file",
+    "write_file",
+    "list_sheets",
+    "detect_encoding",
+    "detect_delimiter",
+    "detect_header_row",
+    # Config
+    "DeduplicationConfig",
+    "StrategyConfig",
+    "ColumnStrategyConfig",
+]