release: v1.8.1 - single-source version, batch backtest detail output

- Consolidate version to pyproject.toml as single source of truth
- __init__.py, cli/__init__.py, docs/conf.py all read dynamically
- run_all_strategies.py now shows best strategy full trade details
- Update README changelog for 1.8.1

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/easy_tdx/__init__.py

@@ -107,4 +107,11 @@ __all__ = [
     "save_best_ex_host",
 ]
 
-__version__ = "1.8.0"
+
+def _get_version() -> str:
+    from importlib.metadata import version
+
+    return version("easy-tdx")
+
+
+__version__ = _get_version()

src/easy_tdx/cli/__init__.py

@@ -23,7 +23,7 @@ from ..backtest.cli import backtest
 
 
 @click.group()
-@click.version_option(version="1.8.0", prog_name="easy-tdx")
+@click.version_option(version=__import__("importlib.metadata", fromlist=["version"]).version("easy-tdx"), prog_name="easy-tdx")
 def cli() -> None:
     """easy-tdx -- 通达信行情数据 CLI（默认 JSON 输出，适合 Agent 使用）。