Migrate from changelog_entry.yaml to towncrier fragments

[MaxGhenis]

Summary

• Replace changelog_entry.yaml with towncrier fragment-based changelog system
• Each PR creates a fragment file in changelog.d/ instead of editing a shared file
• This eliminates merge conflicts when multiple PRs are open
• Add bump_version.py for automated semver inference from fragment types
• Update CI workflows for fragment validation and towncrier build

Test plan

• Verify check-changelog CI job passes
• Verify lint passes
• Test towncrier build locally

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