Document exported public API surface

[codeforester]

Summary

• document the exported history, command-protocol, and context symbols
• add a regression guard requiring docstrings on exported public callables and classes
• record the public API documentation improvements in the unreleased changelog

The affected modules are part of the supported public surface. Explicit
docstrings make generated API references useful and the test guard prevents
future exported symbols from silently shipping without documentation.

Fixes #179

Validation

• .venv/bin/python -m pytest -q (404 passed)
• .venv/bin/python -m pytest -q tests/test_public_api.py (6 passed)
• .venv/bin/python -m mypy --strict lib/python/base_cli
• .venv/bin/ruff format --check lib/python/base_cli/history.py lib/python/base_cli/command_protocol.py lib/python/base_cli/context.py tests/test_public_api.py
• .venv/bin/ruff check lib/python/base_cli/history.py lib/python/base_cli/command_protocol.py lib/python/base_cli/context.py tests/test_public_api.py
• .venv/bin/python scripts/validate_docs.py
• .venv/bin/python scripts/validate_examples.py
• git diff --check