docs: clarify package smoke contract

[CoderDeltaLAN]

Summary:

• Clarifies that package-level execution with python -m agent_rules_kit is not part of the current smoke contract unless main.py is added later.
• Keeps the verified module execution contract as python -m agent_rules_kit.cli.
• Updates the changelog for the documentation clarification.

Verification:

• PYTHONPATH=src python -m agent_rules_kit.cli --version passed.
• PYTHONPATH=src python -m agent_rules_kit.cli check ... --format json passed.
• Full local check passed: 118 tests, Ruff, text hygiene, git whitespace.

Scope:

• CHANGELOG.md
• docs/V0.2-PACKAGING-DRY-RUN.md