refactor: generate train and serve CLIs from config-backed option specs

- Add astrai/config/cli.py: OptSpec tables plus apply_specs infer click types and defaults from config fields, covering Optional[X], Union[X, None], PEP 604 X | None, stringified PEP 563 annotations, bool flag pairs, and repeatable list options
- Move GroupedCommand/GroupedOption and the three-layer YAML merge (option defaults < YAML < explicit CLI) into the config package, adding unknown-key warning and mapping validation
- Replace ~420 lines of hand-written @opt decorators in scripts/tools/train.py with a 66-entry spec table; option names, defaults, flag styles, and YAML semantics verified unchanged
- Migrate scripts/tools/server.py to the same mechanism with its section binding, integer coercion, and dtype validation preserved locally
- Add tests/config/test_cli.py covering type inference across annotation styles, default overrides, flag pairs, merge precedence, scientific notation, and help ordering

astrai/config/__init__.py

@@ -1,3 +1,11 @@
+from astrai.config.cli import (
+    GroupedCommand,
+    GroupedOption,
+    OptSpec,
+    apply_specs,
+    merge_yaml_into_kwargs,
+    opt,
+)
 from astrai.config.model_config import (
     AutoRegressiveLMConfig,
     BaseModelConfig,
@@ -22,4 +30,10 @@ __all__ = [
     "OutputConfig",
     "PipelineConfig",
     "ProcessingConfig",
+    "GroupedCommand",
+    "GroupedOption",
+    "OptSpec",
+    "apply_specs",
+    "merge_yaml_into_kwargs",
+    "opt",
 ]