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

uv.lock

@@ -1154,7 +1154,7 @@ wheels = [
 
 [[package]]
 name = "retro-gamer"
-version = "0.2.0"
+version = "0.3.0"
 source = { editable = "." }
 dependencies = [
     { name = "click" },
@@ -1196,7 +1196,7 @@ documentation = [
 
 [[package]]
 name = "retro-games"
-version = "2.4.1"
+version = "2.5.0"
 source = { editable = "../retro" }
 dependencies = [
     { name = "blessed" },
@@ -1207,8 +1207,8 @@ requires-dist = [{ name = "blessed", specifier = ">=1.33.0" }]
 
 [package.metadata.requires-dev]
 documentation = [
-    { name = "furo", specifier = ">=2025.12.19" },
     { name = "sphinx", specifier = ">=8.1.3" },
+    { name = "sphinx-rtd-theme", specifier = ">=3.0" },
 ]
 
 [[package]]