release: prepare base-cli 0.4.0

[codeforester]

Closes #50.

Summary

• Set the package version and changelog to 0.4.0.
• Document the pre-1.0 compatibility boundary and migration notes.
• Mark the 0.4.x support line and update adopter-facing examples.
• Make the release-candidate security gate audit third-party dependencies without requiring the unpublished project distribution from PyPI.

Validation

• Full base-cli test suite passes.
• Documentation, examples, consumer, artifact, and installed-wheel validators pass.
• Cross-platform CI matrix is expected to run against this issue-backed branch.

Release order

1. Merge this PR.
2. Tag and publish base-cli 0.4.0.
3. Merge the dependent Base alignment PR after its checks pass against the published release.

The Base consumer alignment is tracked in basefoundry/base#1875.