chore: release 0.4.4

[rlippmann]

What changed:

• Made engine.state read-only by removing the setter.
• Kept state initialization/replacement via constructor input and import_json(payload).
• Added public read helpers:
  • get_focus_value(state)
  • get_prohibited_items(state)
• Exported helper APIs from the package root.
• Refactored demos/examples to use helper accessors instead of direct nested state-key reads.
• Updated README and spec docs to align state-access guidance with the public API.
• Added targeted tests for:
  • read-only engine.state
  • helper behavior and defensive-copy semantics
  • clarify-without-mutation on mixed correction-style input.
• Bumped package version to 0.4.4 in pyproject.toml and uv.lock.
• Updated AGENTS PR guidance to clarify that a dedicated "Validation" section in PR text is optional.

Why the change was needed:

• Tighten the public state API around explicit read-only access.
• Provide stable convenience accessors for host integrations.
• Reduce coupling to nested state layout in user-facing examples/docs.
• Package the changes as the 0.4.4 release.