Multi-project comparison comment

[matejchalk]

User story

When running Code PushUp separately on different projects in a monorepo, I want a single comparison comment in my PRs which summarizes changes to all affected projects.

Acceptance criteria

• [x] implement CLI command to merge multiple report diffs into single markdown file
• [x] markdown has sections per project
• [x] project name is included in each section heading
• [x] more compact layout - only categories table shown by default, groups and audits in expandable details
• [ ] code-pushup/github-action#35

Related links

• https://github.com/code-pushup/cli/pull/706#discussion_r1660478690