feat: ai-changelog integration — generate changelog on merge

[hermanngeorge15]

Description

Integrate with @unityinflow/ai-changelog to automatically generate changelog entries when PRs merge to main.

Acceptance Criteria

• Optional generate-changelog input (default: false)
• On PR merge, call ai-changelog to generate entry from PR diff
• Append entry to CHANGELOG.md or create if missing
• Skip if generate-changelog is false

Milestone

v0.1.0