fix: filter no-op dependency update bullets

[derogab]

Summary

Prevent changelog generation from publishing placeholder dependency update entries when no dependencies actually changed.

Changes

• Tighten the changelog prompt so dependency update bullets are only generated when dependency names are known.
• Add a dedicated changelog filter pipeline that removes no-op dependency bullets before markdown normalization.
• Keep markdown normalization focused on formatting by proving it does not filter changelog content.
• Cover the new filtering behavior with unit tests.
• Update README action version references to v2.0.4.

Test plan

• python3 -m unittest discover -s tests
• git diff --check