Add skill diff command

[devrimcavusoglu]

Context

There's no way to compare two skills or detect drift between a registry skill and its installed copy on a platform.

Proposed Changes

• Add skern skill diff <name> command
• Compare registry skill against installed copy on a platform
• Compare two skills by name: skern skill diff <name-a> <name-b>
• Show differences in metadata and body content
• Support --json output

References

Analysis item 2.8