feat: add generic package skeleton

[NWarila]

PT-M3 - Minimal generic green skeleton

Ships the renamable src/sample_app/ package ADR-0004 specifies, making "use template -> rename -> green CI" true.

What

• 8-file anatomy per ADR-0004: __init__ (curated API + __all__), __main__ (python -m self-demo), py.typed, config.py (frozen pydantic-settings: BaseSettings + env-prefix + load()/save_defaults()), exceptions.py (typed What/Why/Fix), _contracts.py (frozen models + Result[T] + StrEnum codes), validators.py (pure, wired via model_validator), main.py (CLI + worked function).
• tests/test_sample_app.py at >=90% coverage (incl. invalid-repeat covered end-to-end).
• pyproject: src-layout discovery, py.typed package-data, pydantic + pydantic-settings deps, pydantic.mypy plugin, --cov=src. Build backend unchanged (setuptools).
• ADR-0004 Implementing-PRs + Changelog updated.

Verified

• Fresh clone -> README rename -> qa.py 6/6 green, no manual fixups.
• python -m <pkg> self-demo works; py.typed ships in the built wheel; pydantic.mypy active (bad field fails mypy).
• Zero framework/domain leakage; scope limited to src/, tests/, pyproject, README, ADR-0004.