release: prepare base-cli 0.4.0

[codeforester]

Summary

• Promote the completed adoption, lifecycle, integration, security, and contract work into the 0.4.0 release.
• Update the release version, changelog, security support window, and migration guidance.

Compatibility

This is a pre-1.0 minor release and therefore a compatibility boundary. Consumer profiles should use ConfigurationError for expected configuration failures, consumers should import RuntimeLayout from the public base_cli.runtime module, and native --option=value syntax is now supported.

Validation

• Full base-cli test suite passes.
• Package, sdist, documentation, examples, and downstream consumer validation pass.
• 0.4.0 wheel/sdist validation passes.
• Installed-wheel public API smoke test passes.

Release

After merge, create the matching v0.4.0 tag and publish through the protected PyPI workflow. Merge the dependent Base alignment PR after the package is available on PyPI.

[codeforester]

Superseded by the issue-backed canonical release PR #101 so the repository branch-policy check can validate the train.