feat(cli): add diff command to compare two versions

[BryanFRD]

Package
Which package does this affect? [x] cli [ ] schema [ ] docs [ ] ci

Problem / Motivation
See title.

Proposed solution
To be defined — see Problem / Motivation above.

Alternatives considered
None considered explicitly — this is the natural shape.

Additional context
Description

Add a ferrflow diff command that shows a detailed comparison between two versions of a package.

Usage

ferrflow diff api v1.4.0..v1.6.0

Output

• List of commits between the two versions with their bump type
• Files changed (summary)
• Changelog that would be generated for the range
• Breaking changes highlighted

Use cases

• Auditing what went into a release
• Debugging why a version was bumped a certain way
• Generating release notes retroactively for a range of versions