Publish API stability and deprecation policy (#69)

[codeforester]

Summary

• publish the public API stability, SemVer, support-window, and experimental-namespace policy
• add a typed base_cli.deprecated() decorator and BaseCliDeprecationWarning
• add migration guidance and contract tests for facade exports, protocol framing, and JSON v1 schemas

Why

Adopters need explicit compatibility promises and a predictable deprecation path before depending on
base-cli in professional tooling. The policy makes the pre-1.0 boundary clear while documenting the
stricter 1.0 contract, and the tests turn the public surface and versioned schemas into release gates.

Closes #69

Validation

• ./tests/validate.sh
• uv run --extra dev --extra typer pytest -q
• ruff format --check scripts examples
• ruff check lib/python/base_cli scripts examples tests
• mypy --strict examples/typed_consumer.py
• python scripts/validate_docs.py
• python scripts/benchmark_runtime.py --check
• pytest --cov=base_cli --cov-fail-under=80 (84.1%)
• bandit -q -r lib/python/base_cli scripts -lll -iii
• pip-audit --strict