[Workflows] Changelog on PR and commits to develop

[ashleyshaw]

Title: [Workflows] Changelog on PR and commits to develop
Labels: area:workflows, v0.2.0, changelog
Summary: Ensure CHANGELOG.md updates on PRs and direct commits to develop only.
Acceptance Criteria:

changelog.yml triggers: pull_request, push (branch: develop).

Fails PR if entry missing; appends on merge.
Impact/Risk: Accurate release notes without branch noise.
Owner: Workflows
Telemetry: % PRs merged with a valid changelog entry.