Refactor CLI/interface: init command, factory field, remove extras_size

- Rename `retro-gamer create` to `retro-gamer init` with positional
  args (GAME OUTPUT) instead of --game/--output flags
- Add [tool.retro-gamer].factory = "module:attr" to declare the game
  factory function from pyproject.toml instead of relying on a
  hard-coded create_game attribute
- Remove extras_size from user-facing config; it is now measured
  automatically from a sample observation and never declared
- Update docs throughout: create→init, runs/→training/, add factory
  field documentation, clarify [model] vs [training] hyperparameter
  sections, remove stale version-history explanations
- Bump version to 0.3.0; require retro-games>=2.5.0

retro_gamer/cli.py

@@ -22,15 +22,12 @@ def cli():
 
 
 # ---------------------------------------------------------------------------
-# retro-gamer create
+# retro-gamer init
 # ---------------------------------------------------------------------------
 
 @cli.command()
-@click.option('--game', required=True,
-              help='Game to train: a .py file path (e.g. my_game.py) or a Python module '
-                   '(e.g. retro.examples.snake)')
-@click.option('--output', required=True,
-              help='Directory to create for this training run')
+@click.argument('game')
+@click.argument('output')
 @click.option('--learning-rate', default=DEFAULTS['learning_rate'], type=float,
               help=f"Adam optimizer learning rate (default {DEFAULTS['learning_rate']})")
 @click.option('--learning-rate-decay', default=DEFAULTS['learning_rate_decay'], type=float,
@@ -60,11 +57,17 @@ def cli():
 @click.option('--prioritize-experiences/--no-prioritize-experiences',
               default=DEFAULTS['prioritize_experiences'],
               help='Use prioritized experience replay')
-def create(game, output, **hyperparams):
+def init(game, output, **hyperparams):
     """Create a new training run directory.
 
-    Game metadata (actions, reward signal, etc.) is read from the
-    [tool.retro-gamer] section of the game's pyproject.toml.
+    GAME is a path to a game directory or a Python module name
+    (e.g. games/snake or retro.examples.snake).
+
+    OUTPUT is the directory to create for this training run
+    (e.g. training/snake).
+
+    Game metadata (actions, reward signal, factory function, etc.) is
+    read from the [tool.retro-gamer] section of the game's pyproject.toml.
     Board size is read directly from the game. Hyperparameter options
     control how the trainer learns, not what it learns about.
     """
@@ -84,6 +87,9 @@ def create(game, output, **hyperparams):
     except (FileNotFoundError, ValueError) as e:
         raise click.ClickException(str(e))
 
+    if metadata.factory:
+        game_config['factory'] = metadata.factory
+
     game_factory = _load_factory(game_config)
     if metadata.board_size is None:
         g = game_factory()
@@ -105,7 +111,7 @@ def create(game, output, **hyperparams):
     with open(run_dir / 'config.toml', 'wb') as f:
         tomli_w.dump(config, f)
 
-    click.echo(f"Created training run at {output}/config.toml")
+    click.echo(f"Initialized training run at {output}/config.toml")
     click.echo(f"  game        : {game}")
     click.echo(f"  board_size  : {metadata.board_size[0]}×{metadata.board_size[1]}")
     click.echo(f"  actions     : {metadata.actions}")
@@ -370,6 +376,21 @@ def _load_factory(game_config: dict):
     path = game_config.get('path')
     if path and path not in sys.path:
         sys.path.insert(0, path)
+
+    factory_str = game_config.get('factory')
+    if factory_str:
+        module_name, attr_name = factory_str.split(':', 1)
+        try:
+            module = importlib.import_module(module_name)
+        except ImportError as e:
+            raise click.ClickException(f"Cannot import factory module '{module_name}': {e}")
+        if not hasattr(module, attr_name):
+            raise click.ClickException(
+                f"Module '{module_name}' has no '{attr_name}' function "
+                f"(from factory = {factory_str!r})"
+            )
+        return getattr(module, attr_name)
+
     module_name = game_config['module']
     try:
         module = importlib.import_module(module_name)
@@ -377,7 +398,8 @@ def _load_factory(game_config: dict):
         raise click.ClickException(f"Cannot import game module '{module_name}': {e}")
     if not hasattr(module, 'create_game'):
         raise click.ClickException(
-            f"Module '{module_name}' has no create_game() function"
+            f"Module '{module_name}' has no create_game() function. "
+            "Add factory = \"module:create_game\" to [tool.retro-gamer] in pyproject.toml."
         )
     return module.create_game