typing: ship a `py.typed` marker so downstream mypy/pyright actually see our type hints (PEP 561)

[Koukyosyumei]

Problem

The SDK is fully type-annotated — e.g. src/h5i/orchestra/_errors.py (def __init__(self, message: str, code: int, kind: str | None = None)), _conductor.py, policy.py — but the package ships no py.typed marker:

$ find src -name py.typed
(nothing)

Per PEP 561, without that marker, type checkers in downstream projects treat h5i.orchestra as untyped and silently ignore every annotation we wrote. Users of mypy/pyright get no completions-with-types, no signature checking, nothing — the annotations only benefit our own test suite.

Suggested fix

• Add an empty src/h5i/py.typed file (the package root that [tool.hatch.build.targets.wheel] ships is src/h5i).
• Verify the marker lands in the wheel: python -m build && unzip -l dist/*.whl | grep py.typed.
• Bonus: add a tiny smoke check to CI (or docs) showing mypy resolves types from the installed package, e.g. mypy -c "from h5i.orchestra import Conductor; reveal_type(Conductor)".

The core change is literally one empty file plus a build check — a perfect first contribution — but it meaningfully improves the experience of every typed downstream project.